Alkaline cell with flat housing and nickel oxyhydroxide cathode

ABSTRACT

An alkaline cell having a flat casing, preferably of cuboid shape. The cell can have an anode comprising zinc and a cathode comprising nickel oxyhydroxide. The casing can have a relatively small overall thickness, typically between about 5 and 10 mm, but may be larger. Cell contents can be supplied through an open end in the casing and an end cap assembly inserted therein to seal the cell. The end cap assembly includes a vent mechanism, preferably a grooved vent, which can activate, when gas pressure within the cell reaches a threshold level typically between about 250 and 800 psig (1724×10 3  and 5515×10 3  pascal gage). The cell can have a supplemental vent mechanism such as a laser welded region on the surface of the casing which may activate at higher pressure levels.

[0001] CROSS REFERENCE TO RELATED APPLICATIONS

[0002] This application is a continuation in part of application Ser.No. 10/722,879 filed Nov. 26, 2003 which is continuation in part of Ser.No. 10/414,750 filed Apr. 16, 2003, which is a continuation in part ofapplication Ser. No. 10/336261 filed Jan. 3, 2003.

FIELD OF THE INVENTION

[0003] The invention relates to an alkaline battery having asubstantially flat outer housing. The invention relates to alkalinebattery having an anode comprising zinc, a cathode comprising nickeloxyhydroxide, and an electrolyte comprising aqueous potassium hydroxide.

BACKGROUND

[0004] Conventional alkaline electrochemical cells have an anodecomprising zinc and a cathode comprising manganese dioxide. The cell istypically formed of a cylindrical outer housing. The fresh cell has anopen circuit voltage (EMF) of about 1.5 volt and typical average runningvoltage of between about 1.0 to 1.2 Volt in medium drain service (100 to300 milliamp). The cylindrical housing is initially formed with anenlarged open end and opposing closed end. After the cell contents aresupplied, an end cap assembly with insulating grommet and negativeterminal end cap is inserted into the housing open end. The open end isclosed by crimping the housing edge over an edge of the insulating plugand radially compressing the housing around the insulating plug toprovide a tight seal. The insulating grommet electrically insulates thenegative end cap from the cell housing. A portion of the cell housing atthe opposing closed end forms the positive terminal.

[0005] A problem associated with design of various electrochemicalcells, particularly alkaline cells, is the tendency of the cell toproduce gases as it continues to discharge beyond a certain point,normally near the point of complete exhaustion of the cell's usefulcapacity. Electrochemical cells, particularly alkaline cells, areconventionally provided with rupturable diaphragms or rupturablemembranes within the end cap assembly. The rupturable diaphragm ormembrane may be formed within a plastic insulating member as described,for example, in U.S. Pat. No. 3,617,386.

[0006] A cylindrical alkaline cell comprising a zinc anode and nickeloxyhydroxide cathode is disclosed in commonly assigned patentapplication U.S. Ser. No. 10/228,957 filed Aug. 28, 2002, Pub. No.US2004/0043292A1. Such a cell generally produces less hydrogen gas andconsequently has lower internal pressure after discharge than aconventional alkaline cell with a comparable zinc anode. Commonlyassigned patent application U.S. Ser. No. 10/831,899, filed Apr. 26,2004 discloses anode and cathode compositions for alkaline cellscomprising zinc anode and nickel oxyhydroxide cathode related tocompositions herein in the present application.

[0007] The prior art discloses rupturable vent membranes, which areintegrally formed as thinned areas within the insulating disk includedwithin the end cap assembly. Such vent membranes can be oriented suchthat they lie in a plane perpendicular to the cell's longitudinal axis,for example, as shown in U.S. Pat. No. 5,589,293, or they may beoriented so that they are slanted in relation to the cell's longitudinalaxis as shown in U.S. Pat. No. 4,227,701. U.S. Pat. No. 6,127,062discloses an insulating sealing disk and an integrally formed rupturablemembrane, which is oriented vertically, that is, parallel to the cell'scentral longitudinal axis. When the gas pressure within the cell risesto a predetermined level the membrane ruptures thereby releasing the gaspressure to the external environment through apertures in the end cap.

[0008] Other types of vents are disclosed in the art for relieving gaspressure within an electrochemical cell. One such vent is a reseatablerubber plug, which has been used effectively in connection with smallflat rectangular shaped nickel metal hydride rechargeable cells. Onesuch rechargeable battery with the reseatable rubber plug vent is a7/5-F6 size nickel metal hydride rechargeable battery availablecommercially as battery model GP14M145 manufactured by Gold PeakBatteries, Hong Kong. The rubber plug is physically compressed to sittightly within a beveled aperture within a cavity or seat in the cell'send cap assembly. When the cell's internal gas pressure reaches apredetermined level, the plug lifts off its seat thereby letting gas toescape through the underlying aperture. The plug reseats itself when thegas pressure within the cell returns to normal. Primary alkalineelectrochemical cells typically include a zinc anode active material, analkaline electrolyte, a manganese dioxide cathode active material, andan electrolyte permeable separator film, typically of cellulose orcellulosic and polyvinylalcohol fibers. The anode active material caninclude for example, zinc particles admixed with conventional gellingagents, such as sodium carboxymethyl cellulose or the sodium salt of anacrylic acid copolymer, and an electrolyte.

[0009] The gelling agent serves to suspend the zinc particles and tomaintain them in contact with one another. Typically, a conductive metalnail inserted into the anode active material serves as the anode currentcollector, which is electrically connected to the negative terminal endcap. The electrolyte can be an aqueous solution of an alkali metalhydroxide, for example, potassium hydroxide, sodium hydroxide, lithiumhydroxide or a mixture thereof. The cathode can include a particulatemetal oxide, for example, manganese dioxide, nickel oxyhydroxide or amixture thereof, as the electrochemically active material admixed withan electrically conductive additive, typically a carbonaceous material,such as a graphitic carbon, for example, graphite to enhance the bulkelectrical conductivity. Optionally, a small amount of a polymericbinder, for example, polyethylene or polytetrafluoroethylene, and otherperformance enhancing additives, such as a titanium-containing compoundcan be included in the cathode.

[0010] Suitable cathode active materials can be selected from amanganese dioxide, preferably an electrolytic manganese dioxide (EMD)made by direct electrolysis of an acidic solution of manganese sulfate;a nickel oxyhydroxide, for example, beta-nickel oxyhydroxide orgamma-nickel oxyhydroxide; and physical mixtures of electrolyticmanganese dioxide and nickel oxyhydroxide. In general, the electricalconductivities of both EMD and nickel oxyhydroxide are relatively low.Therefore, an electrically conductive additive is included in thecathode mixture to improve electric conductivity between individualparticles of the active cathode material as well as between particles ofthe active cathode material and the inner surface of the cylindricalcell housing that also serves as the cathode current collector. Suitableelectrically conductive additives can include carbonaceous materials,for example, graphite, graphitic carbons, conductive carbon powders,such as carbon blacks, including acetylene black and petroleum coke.Desirably the conductive additive can be a graphitic carbon, forexample, a flaky, highly crystalline natural graphite, a flaky, highlycrystalline synthetic graphite, an expanded or exfoliated graphite,graphitized carbon fibers, carbon nanofibers, a fullerene or mixturesthereof. Preferably the graphitic carbon is an oxidation-resistantgraphite.

[0011] There are small size rectangular shaped rechargeable batteriesnow available commercially, that can be used to power small electronicdevices such as MP3 audio players and mini disk (MD) players. Thesebatteries typically have a small cuboid (rectangular parallelepiped)shape and are similar in size to a package of stick-type chewing gum.The term “cuboid” as used herein shall mean its normal geometricaldefinition, namely, a “rectangular parallelepiped”. Such batteries, forexample, can be in the form of replaceable rechargeable nickel metalhydride (NiMH) size F6 or 7/5F6 size cuboids in accordance with thestandard size for such batteries as set forth by the InternationalElectrotechnical Commission (IEC). The F6 size has a thickness of 6.0mm, width of 17.0 mm and length of 35.7 mm (without label). There is aversion of the F6 size wherein the length can be as great as about 48.0mm. The 7/5-F6 size has thickness of 6.0 mm, width of 17.0 mm, andlength of 67.3 mm. According to the IEC standard, allowed deviation forthe 7/5-F6 size in thickness is +0 mm, −0.7 mm, in width is +0 mm, −1mm, and in length is +0, −1.5 mm. The average running voltage of the F6or 7/5F6 NiMH rechargeable batteries when used to power miniaturedigital audio players such as an MP3 audio player or mini disk (MD)players is between about 1.1 and 1.4 volt typically about 1.12 volt.

[0012] When used to power a mini disk (MD) player, the battery isdrained at a rate of between about 200 and 250 milliAmp. When used topower a digital audio MP3 player the battery is drained typically at arate of about 100 milliAmp.

[0013] It would be desirable to have a small flat alkaline primary(i.e., non-rechargeble) battery of the same size and shape as the smallsize cuboid-shaped (i.e., rectangular parallelepiped) rechargeablenickel metal hydride batteries, so that the small size alkaline primarybattery could be used interchangeably with a rechargeable nickel metalhydride battery to power small electronic devices such as mini diskplayers, MP3 players, and handheld games.

[0014] It would be highly desirable to use a small rectangular-shapedprimary alkaline battery, preferably one having a cathode includingnickel oxyhydroxide to replace small rectangular-shaped rechargeablebatteries, particularly small nickel metal hydride rechargeablebatteries.

[0015] For a given cell housing (casing) wall thickness, it will beappreciated that a rectangular-shaped cell housing is less able towithstand any increase in internal pressure of the cell (due to gassingand cathode expansion) than a cylindrical-shaped housing of comparablesize and internal volume. This is due to the significantly greatercircumferential stress (hoop stress) imposed on a rectangular (i.e.,cuboid) shaped housing than on a similar size cylindrical housing forany given pressure and housing wall thickness. The problem ofdeformation (i.e., bulging or swelling) associated withrectangular-shaped cells can be overcome by significantly increasing thewall thickness of the housing. However, a significant increase inhousing wall thickness can result in significant decrease in availablevolume for anode and cathode materials for rectangular cells havingsmall overall thickness, e.g. under about 10 mm. The added wallthickness adds to the cost of manufacture of the cell. In this regard itis desirable to keep the housing wall thickness below about 0.50 mm,preferably less than about 0.47 mm.

[0016] Thus it is desired to design a small flat, primary(non-rechargeable) alkaline cell, such as an F6 or 7/5-F6 size cellhaving a rectangular (cuboid) shaped housing, but yet with small housingwall thickness, wherein the housing does not significantly bulge orswell during normal cell usage.

[0017] Thus, it is desired that such a rectangular-shaped primaryalkaline cell be used as a replacement for a same size flat rechargeablenickel metal hydride cell.

SUMMARY OF THE INVENTION

[0018] A principal aspect of the invention is directed toward a primary(non-rechargeable) alkaline cell that can generate less hydrogen gasupon discharge than conventional alkaline cells, wherein said cell hasan outer casing (housing), an end cap assembly including a ventmechanism to allow the hydrogen gas to escape from the cell wheninternal pressure of the cell reaches a predetermined level. The cellcasing has at least one pair of opposing flat walls running along thecell's length.

[0019] The alkaline cell can have the shape of a parallelepiped, butdesirably has the shape of a cuboid (i.e., rectangular parallelepiped).Preferably, the cell casing can be cuboid in shape, and does not includeany integral cylindrical cross-sections. The alkaline cell desirablyincludes an anode comprising zinc-based particles, a cathode comprisingnickel oxyhydroxide, and an alkaline electrolyte, preferably an aqueouspotassium hydroxide solution.

[0020] In another principal aspect of the invention, the flat alkalinecell can include a cathode, comprising nickel oxyhydroxide(NiOOH), ananode, comprising zinc or a zinc-based alloy, a separator positionedbetween anode and cathode, and an alkaline electrolyte solution,comprising potassium hydroxide, contacting both anode and cathode.

[0021] In one aspect of the invention, the cathode comprises nickeloxyhydroxide, a conductive additive such as a graphitic carbon,preferably an oxidation-resistant graphite, and an aqueous alkalineelectrolyte solution. Optionally, the cathode can include a polymericbinder, an oxidizing additive or combinations thereof. The cathode caninclude, for example, between 60% and 97% by weight, between 80% and 95%by weight, or between 85% and 90% by weight of nickel oxyhydroxide.

[0022] The nickel oxyhydroxide can include beta-nickel oxyhydroxide,cobalt oxyhydroxide-coated beta-nickel oxyhydroxide, gamma-nickeloxyhydroxide, cobalt oxyhydroxide-coated gamma-nickel oxyhydroxide, amixture of beta-nickel oxyhydroxide and gamma-nickel oxyhydroxide or amixture of cobalt oxyhydroxide-coated beta-nickel oxyhydroxide andcobalt oxyhydroxide-coated gamma-nickel oxyhydroxide. The nickeloxyhydroxide can be a powder including particles that are nominallyspherical, spheroidal, or ellipsoidal in shape. The average particlesize of the nickel oxyhydroxide powder can range between 2 and 50microns, 5 and 30 microns, 10 and 25 microns or 15 and 20 microns. Thenickel oxyhydroxide can include at least one bulk dopant. A bulk dopantcan increase conductivity of the nickel oxyhydroxide as well as decreaseits half-cell potential thereby decreasing open circuit voltage (OCV) ofthe cell.

[0023] In another aspect of the invention, the cathode can include anelectrically conductive additive capable of enhancing bulk electricalconductivity of the cathode. Examples of suitable electricallyconductive additives include conductive carbon particles, nickel powder,cobalt powder, cobalt oxide, cobalt oxyhydroxide, carbon fibers, carbonnanofibers, or combinations thereof. Carbon nanofibers are described,for example, in commonly assigned U.S. Ser. No. 09/658,042, filed Sep.7, 2000 and U.S. Ser. No. 09/829,709, filed Apr. 10, 2001. Moreparticularly, the cathode can include between 2 and 20 wt. %, between 5and 15 wt. % or between 6 and 8 wt. % of conductive carbon particles.Conductive carbon particles can include graphitized carbon, carbonblack, acetylene black or petroleum coke. The conductive carbonparticles can have a variety of shapes including substantiallyspherical; elongated having one dimension substantially longer than theothers; flake-like having two dimensions elongated relative to a third;and fibrous or thread-like. Preferably, the conductive carbon is agraphitized carbon. Graphitized carbon can include natural graphite,synthetic graphite, expanded graphite, graphitized carbon black,graphitic carbon nanofibers, fullerenes or mixtures thereof. Typically,natural and synthetic graphite particles can have a flake-like shape.Further, by including a graphitized carbon in the cathode at arelatively high level, for example, 6 to 10 wt. %, the capacity of cellsdischarged at low drain rates after storage can be increased byincreasing the cathode efficiency.

[0024] In an alkaline cell including nickel oxyhydroxide as the activecathode material, it is preferable to use a natural or syntheticgraphite that is oxidation-resistant. Typically, an oxidation-resistantgraphite can be prepared by treating a high purity natural or syntheticgraphite in an inert atmosphere at very high temperatures, for example,at temperatures greater than about 2500° C. or greater than about 3000°C. Cells with cathodes containing nickel oxyhydroxide can have an opencircuit voltage (OCV) sufficiently high so as to oxidize alkalineelectrolyte thereby generating oxygen gas. Oxidation of alkalineelectrolytes by nickel oxyhydroxide is a well-known self-dischargeprocess of charged nickel electrodes. The evolved oxygen gas can promoteoxidation of the graphite present in the cathode thereby decreasingcathode conductivity and forming soluble potassium carbonate as aby-product. An increase in the carbonate ion concentration in theelectrolyte can decrease ionic conductivity. Also, during storage atelevated temperatures, nickel oxyhydroxide can oxidize graphitedirectly.

[0025] In another aspect of the invention, it has been determined thatuse of an oxidation-resistant graphite in cells with cathodes includingnickel oxyhydroxide can greatly reduce the extent of the undesirableself-discharge processes. In general, the oxidation resistance of agraphite can be determined by many contributing factors. For example, itis believed that the rate of graphite oxidation is at least partiallyrelated to the specific surface area of the graphite particles wherebythe smaller the surface area, the more oxidation resistant the graphite.Similarly, oxidation resistance of a graphite can be at least partiallyrelated to the average particle size and the particle size distribution.Because larger size particles typically have lower surface areas, theycan be more oxidation resistant. Also, oxidation resistance is believedto be related at least partially to the average crystallite size of thegraphite particles as determined by x-ray diffraction, whereby thelarger the crystallite size, the more oxidation-resistant the graphite.Further, it is believed that oxidation resistance also can depend atleast partially on the relative number of surface defects ordislocations present in the graphite particles. Namely, the fewer therelative number of defects, the more oxidation-resistant the graphite.

[0026] In one aspect of the invention, the flat alkaline cell includes acathode, an anode, a separator between anode and cathode and an alkalineelectrolyte contacting both anode and cathode. The cathode preferablyincludes nickel oxyhydroxide and an oxidation-resistant graphite. Theanode can include, for example, between about 60 and 80 wt. %, betweenabout 62 and 75 wt. % or between about 62 and 72 wt. % of zinc-basedparticles comprising zinc or zinc alloy particles. The term “zinc” asused herein shall be understood to include “zinc alloy” which comprisesa very high concentration of zinc and as such functionselectrochemically essentially as pure zinc. The zinc-based particles canhave a mean average particle size, for example, between about 1 and 350microns, desirably between about 1 and 250 microns, preferably betweenabout 20 and 250 microns. Particle size and mean average particle size,as reported herein, unless otherwise specified, shall be construed to bethat determined by the laser diffraction method. The zinc-basedparticles can be generally acicular, flake-like or spherical in shape.

[0027] In another principal aspect of the invention, a method forimproving the discharge performance of a flat alkaline cell afterstorage includes providing a cathode including an active cathodematerial comprising nickel oxyhydroxide and a conductive additivecomprising an oxidation-resistant graphite, an anode includingzinc-based particles comprising zinc or zinc alloy, of which at leastabout 10 wt. % are 200 mesh in size or smaller, and forming a flat cellincluding the cathode and anode.

[0028] A flat zinc/nickel oxyhydroxide cell can have improvedperformance when discharged after storage at high temperature when zincfines are included in the anode. As used herein, “zinc fines” are zincparticles small enough to pass through a sieve of 200 mesh size (i.e., a200 mesh size corresponds to a sieve having square openings of 0.075mm). Typically, zinc fines capable of passing through a 200 mesh sievecan have a mean average particle size of between about 1 and 75 microns.The inclusion of zinc fines in the anode of a flat zinc/NiOOH cell hasbeen determined to improve discharge performance at high drain rates andunexpectedly, at low drain rates. It is believed that addition of zincfines can improve electrical conductivity between the zinc-basedparticles and the current collector as well as increase the totalsurface area of the zinc-based particles in the anode. Since the ratecapability of the nickel oxyhydroxide cathode is high, increasing thesurface area of the zinc-based particles can decrease the effectivecurrent density (i.e., Amperes/cm²) and improve rate capability of thezinc anode, thereby providing improved cell performance.

[0029] In addition to the substantial improvement in performanceprovided by adding zinc fines to the zinc anode, both continuous andintermittent discharge capacities of both fresh and stored zinc/NiOOHflat cells can be increased further by substituting anoxidation-resistant graphite for the conventional natural or syntheticgraphite in the cathode. Also, an oxidation-resistant graphite can beincluded beneficially in the conductive coating applied to the innersurface the cell housing. Thus, the particular combination of anoxidation-resistant graphite in the cathode and zinc fines in the anodeof the zinc/NiOOH flat cells is theorized to be particularly effectiveat delaying onset of polarization at both cathode and anode when thecell is discharged at high drain rates either continuously orintermittently, especially after extended storage at high temperaturesbefore discharge. More specifically, by delaying onset of polarizationof the zinc anode, the continuous as well as intermittent dischargecapacities of both fresh cells and cells that have been stored forextended periods of time, for example, up to one year and even longer,can be increased substantially. Advantageously, the overall improvementin performance provided by the particular combination of zinc fines inthe anode with nickel oxyhydroxide and an oxidation-resistant graphitein the cathode can be obtained without further modifying either anode orcathode such as by changing electrode composition, for example, byintroducing other additives or dopants or by substantially increasingthe design capacity of the zinc/NiOOH flat cell.

[0030] In another aspect, an end cap assembly including a ventingmechanism and preferably a rectangular shaped metallic cover is used toclose the open end of the cell casing after the cell contents areinserted into the casing. The end cap assembly is inserted into the openend of the cell casing and is sealed by crimping or welding to close thecasing. The metallic cover can serve as the negative terminal ifinsulation or an insulating seal member, typically of nylon orpolypropylene, is inserted between the edge of the cover and the casingedge and the edge crimped over the insulating seal member. Preferably,the cover can be laser welded directly to the edge of the cell casing.When the cover is laser welded to the casing edge, a plastic extenderseal is stacked on top of the metal cover, a separate end cap thatserves as the negative terminal is stacked in turn onto the plasticextender that is insulated from the cover, but in electricalcommunication with the anode. Alternatively, a paper washer can besubstituted for the plastic extender. The cell casing serves as thecathode current collector as well as the positive terminal.

[0031] The cathode comprising nickel oxyhydroxide andoxidation-resistant graphite is inserted, preferably in the form of aplurality of compacted slabs or disks. The cathode slabs or disks arepreferably rectangular shaped, each having a central hollow core runningthrough the slab's thickness. The slabs are inserted so that they arestacked one on top of another. The slabs are aligned along the cell'slength, so that their outside surface is in contact with the insidesurface of the casing. The stacked cathode slabs form a central hollowcore running along the cell's longitudinal axis. The central hollow corewithin the stacked cathode slabs forms the anode cavity. The insidesurface of each cathode slab, which defines the central hollow core(anode cavity) within the slab, is preferably a curved surface. Suchcurved inside surface improves the mechanical strength of the slabduring transfer and handling and also provides more uniform contactbetween the electrolyte permeable separator and the cathode. Theseparator is inserted into the central hollow core (i.e., anode cavity)so that the outside surface of the separator abuts and closely contactsthe inside surface of the cathode. An anode slurry comprising zinc-basedparticles is inserted into the anode cavity with the separator providingthe interface between anode and cathode. An elongated anode currentcollector in the end cap assembly is inserted into the anode slurry toprovide electrical communication with the negative terminal. The end capassembly has an insulating sealing member, that insulates the anodecurrent collector from the cell casing.

[0032] The anode cavity preferably has an elongated or oblongconfiguration when the anode cavity is viewed in plan view upon taking across section of a cathode slab along a plane perpendicular to thelongitudinal axis of the cell. The outer casing (housing) is desirablyof steel, preferably of nickel-plated steel. The casing wall thicknessis desirably between about 0.30 and 0.50 mm, typically between about0.30 and 0.45 mm, preferably between about 0.30 and 0.40 mm, moredesirably between about 0.35 and 0.40.

[0033] In one aspect a reusable or reactivatable vent, preferably, areseatable plug vent mechanism may be employed as a primary ventmechanism. The reseatable plug is preferably designed to activate whenthe cell's internal gas pressure reaches a threshold level pressure P1of between about 100 and 300 psig (689.5×10³ and 2069×10³ pascal gage),desirably between about 100 and 200 psig (689.5×10³ and 1379×10³ pascalgage).

[0034] In another aspect one or more spaced apart groove vents,preferably a single continuous groove vent (coined vent) comprising atleast one stamped or scored region on the casing surface may be used asthe primary venting mechanism. The groove vent can be stamped, cut orscored into the casing surface so that the underlying thinned regionruptures at pressure P1 between about 250 and 800 psig (1724×10³ and5515×10³ pascal gage). In such design the reseatable plug may beeliminated. Preferably, the groove vent is located in proximity to theclosed end of the casing in proximity to the positive terminal. Thegroove boundary may be closed or open. The groove can be a straight orsubstantially straight, preferably parallel to a casing edge. The groovewidth is typically small, for example, under about 1 mm. However, theterm “groove” or “groove vent” as used herein is not intended to berestricted to any particular width or shape, but rather encompasses anydepression on the cell's casing resulting in a thinned underlyingmaterial region expected to crack or rupture when gas pressure withinthe cell reaches a target threshold level. By way of nonlimitingexample, in a 7/5-F6 size rectangular cell, there may be a groove ventlocated on the casing wide face and parallel to a wide edge, preferablyin proximity to the casing closed end. There may be a plurality of suchgroove vents, but desirably only a single groove vent, on a wide side ofthe casing and about 8 mm in length and about 5 to 10 mm from the closedend of the casing.

[0035] In such aspect of the invention the thinned material underlyingthe groove vent can be designed to rupture when gas pressure within thecell reaches a design burst pressure P1 of between about 250 and 800psig (1724×10³ and 5515×10³ pascal gage). To achieve such range in burstpressure between about 250 and 800 psig (1724×10³ and 5515×10³ pascalgage), the thinned material underlying groove may have a thicknessbetween about 0.04 and 0.15 mm. Alternatively, the thinned materialunderlying the groove vent can be designed to rupture when gas pressurewithin the cell reaches a design burst pressure of between about 400 and800 psig (2758×10³ and 5515×10³ pascal gage), desirably between about400 and 600 psig (2758×10³ and 4136×10³ pascal gage). To achieve arupture pressure between 400 and 800 psig (2758×10³ and 5515×10³ pascalgage) the thinned material underlying the groove may have a thicknessbetween about 0.07 and 0.15 mm.

[0036] The groove vent may be made by stamping the casing surface with adie, preferably a die having a cutting knife edge. A mandrel is heldagainst the inside surface of the casing as the stamping die punchesinto the casing outside surface. The groove can be conveniently cut in aV shape having equal length sides or in the V shape having unequallength sides. In the former case the sides forming the V shaped groovedesirably form an acute angle of about 40 degrees and in the latter casethey form an angle preferably between about 10 to 30 degrees.

[0037] In conjunction with the groove vent there may be a supplementalventing system comprising one or more laser welds securing the metalcover to the casing. Such welds may be comprise one or both a weak laserweld and a strong laser weld which may be designed to rupture atpressures higher than the rupture pressure P1 of the thinned regionunderlying the groove vent. Such laser welds are preferably made using aNd:Yag laser.

[0038] In an aspect of the invention there may be just one laser weld,namely, a strong weld securing the entire circumferential edge of metalcover to the casing. The strong weld can function as the supplementalventing system designed to rupture under catastrophic conditions, forexample, if the cell were inadvertently subjected to recharging underextremely high current or under extremely abusive conditions causing gasgeneration within the cell to rise abruptly to levels between about 800psig and 2500 psig (5515×10³ and 17235×10³ pascal gage).

[0039] Thus, in a preferred aspect there is at least a single groovevent stamped or cut onto the cell casing. The single groove ventfunctions may function as the cell's primary venting mechanism whereinthe thinned material underlying the groove is designed to rupture if gaswithin the cell rises to a level between about 250 and 800 psig(1724×10³ and 5515×10³ pascal gage), more typically between about 400and 800 psig (2758×10³ and 5515×10³ pascal gage). And there is incombination a supplemental venting system comprising a strong laser weldsecuring the edges of a metal cover to the casing. Such strong laserweld is designed to crack or rupture in the event of a catastrophicsituation wherein gas within the cell rises abruptly to levels betweenabout 800 psig and 2500 psig (5515×10³ and 17235×10³ pascal gage). Insuch case gas within the cell will quickly escape through the ruptureand the cell's internal pressure will immediately return to nominallevels.

[0040] In a specific aspect the flat alkaline cell has the overall shapeof a small cuboid (rectangular parallelepiped), typically having anoutside thickness between about 5 and 10 mm, particularly a thicknessbetween about 5 and 7 mm. The outside thickness is measured by thedistance between the outside surface of opposing sides of the housingdefining the short dimension of the cell. In such embodiment the primary(nonrechargeable) alkaline cell of the invention can be used, forexample, as a replacement for small size flat rechargeable cells. Inparticular such primary alkaline cell can be used as a replacement forsame sized rechargeable nickel metal hydride cells, for example, the7/5-F6 size rectangular rechargeable nickel metal hydride cell.

[0041] The amount of gassing and deformation of the cell casing (i.e.,bulging) can be reduced by adjusting the cell balance in the case of theconventional zinc/MnO₂ chemistry. However, it has been determined thatthere is advantageously less gassing and accompanying deformation of thecell associated with a flat rectangular alkaline cell employing an anodecomprising zinc fines and a cathode comprising nickel oxyhydroxide ascompared to the same cell having an anode comprising zinc-basedparticles and a cathode comprising manganese dioxide. The term“substantially flat” as used herein is intended to include flat ornearly flat casing surfaces which have become deformed somewhat becauseof bulging, for example, due to cell gassing. Cell gassing in alkalinecells, in general, results from the reaction of zinc particles withwater in the electrolyte to produce hydrogen gas. In the zinc/NiOOH cellany hydrogen gas generated is essentially consumed by the nickeloxyhydroxide and any oxygen from self-discharge of nickel oxyhydroxideis consumed by zinc-based particles in the anode to produce zinc oxide.Thus, one effect of including nickel oxyhydroxide in the cathode is areduction in the internal gas pressure in the cell. In this regard, theflat rectangular alkaline cell employing a zinc anode comprising zincfines and a nickel oxyhydroxide cathode can be balanced such that theratio of the theoretical capacity (in mAmp-hr) of nickel oxyhydroxide(based on 292 mAmp-hr per gram NiOOH) divided by the theoreticalcapacity (in mAmp-hr) of zinc (based on 820 mAmp-hr per gram zinc) isabout 1, for example, 0.98.

[0042] The zinc/NiOOH flat alkaline cell of the invention has an opencircuit voltage (fresh) of between about 1.75 and 1.80 Volts and anaverage running voltage of about 1.5 Volts. It has a characteristic flatvoltage profile during discharge for the life of the cell. It generallymaintains the high flat voltage profile even at high drain rates. Inthis regard, it can provide a distinct advantage over zinc/MnO₂ flatalkaline cells with the same configuration (e.g. cuboid) in that itgenerally provides greater service life, over a wide range of low tohigh power applications and in particular, in high power applications.

BRIEF DESCRIPTION OF THE DRAWINGS

[0043]FIG. 1 is perspective view of the flat alkaline cell of theinvention showing the cell's negative terminal end.

[0044]FIG. 1A is a perspective view of the flat alkaline cell of FIG. 1showing the cell's positive terminal end.

[0045]FIG. 2 is a cross sectional view of the cell shown in FIG. 1Ataken along view lines 2-2.

[0046]FIG. 2A is a cross sectional view of the cell shown in FIG. 1taken along view lines 2-2 and showing a modified plug design.

[0047]FIG. 3 is a cross sectional view of the cell shown in FIG. 1Ataken along view lines 3-3.

[0048]FIG. 3A is a cross sectional view of the cell shown in FIG. 1Ataken along view lines 3-3 and showing a modified plug design.

[0049]FIG. 4 is an exploded view of the components comprising the endcap assembly for the flat alkaline cell.

[0050]FIG. 4A is a perspective view of the modified plug shown in FIGS.2A and 3A before it has been compressed into its casing cavity.

[0051]FIG. 4B is a plan view of a cross section of the cell taken in aplane perpendicular to the cell's longitudinal axis along sight lines4B-4B of FIG. 1A to show an elongated anode cavity.

[0052]FIG. 4C is a plan view of a cross section of the cell to showanother embodiment of the elongated anode cavity.

[0053]FIG. 4D is a plan view of a cross section to show a thirdembodiment of the elongated anode cavity.

[0054]FIG. 5 is an exploded view showing installation of the cellcontents and end cap assembly into the cell casing.

[0055]FIG. 6 is a plan view of a metal cover plate shown laser weldedalong its edge with a strong weld and a weak weld to the inside surfaceof the cell's casing.

[0056]FIG. 6A is a plan view of a metal cover plate shown laser weldedalong its edge with a strong weld and a weak weld to the crimped edge ofthe cell's casing.

[0057]FIG. 7 is a view of the cell casing body showing an embodiment ofstrong and weak welds therein.

[0058]FIG. 7A is a view of the cell casing body showing an embodiment ofcurved strong and weak welds therein.

[0059]FIG. 8 is a side view of the cell showing a groove within thecasing body forming an underlying region of thinned material.

[0060]FIG. 8A is a side view of the cell showing a plurality of grooveswithin the casing body wherein each groove forms an underlying region ofthinned material.

[0061]FIG. 9 is a cross sectional view of a representative groove shownin FIGS. 8 and 8A.

[0062]FIG. 10 is a top plan view of the metal cover laser welded to thecell casing shown in FIG. 10A.

[0063]FIG. 10A is a side view of an embodiment of the flat alkaline cellshowing a single grooved vent on a wide side of the casing.

[0064]FIG. 11 is cross sectional view along the wide side of the cellshown in FIG. 10A. FIG. 12 is cross sectional view along the short sideof the cell shown in FIG. 10A.

[0065]FIG. 13 is an exploded view of the components comprising the endcap assembly for the flat cell shown in FIG. 10A.

[0066]FIG. 14 shows fabrication of the separator for the cell of FIG.10A from a single sheet of separator material.

[0067]FIG. 14A shows the process of forming a wrapped separator sheet.

[0068]FIG. 14B shows the finished wrapped separator having an open endand opposing closed end.

[0069]FIG. 15 shows an alternative type of V shaped groove cut from thegrooved vent.

[0070]FIG. 16 shows an embodiment wherein there is a gap between a shortside of the separator and the cathode.

[0071]FIG. 17 shows an alternative embodiment similar to the cell ofFIG. 11 except that a paper washer is stacked over the welded metalcover instead of a plastic extender.

DETAILED DESCRIPTION

[0072] A specific embodiment of the flat alkaline cell 10 of theinvention is shown in FIGS. 1-5. Cell 10 has at least two flat opposingsides, which are parallel to the cell's longitudinal axis. Cell 10 ispreferably of rectangular shape, that is, a cuboid, as shown best inFIGS. 1 and 1A. The term “cuboid” as used herein shall mean thegeometrical definition, which is a rectangular parallelepiped. However,cell 10 can also be a parallelepiped. Outer casing 100 as shown in thefigures preferably is of cuboid shape, thus without having any integralcylindrical sections. Cell 10 typically has a thickness smaller than itswidth and a width smaller than its length. When cell thickness, width,and length are of different dimensions, the thickness will normally beconsidered the smallest of these three dimensions.

[0073] The cell 10 preferably comprises a cuboid shaped casing (housing)100, preferably of nickel-plated steel. The inside surface of casing 100can be coated with a layer of a solvent-based conductive carbon coating,for example, available under the trade designation TIMREX® E-LB. In theembodiment shown in the figures, casing 100 is bounded by a pair ofopposing large flat walls 106 a and 106 b; a pair of opposing small flatwalls 107 a and 107 b; a closed end 104; and opposing open end 102. Thecell's thickness is defined by the distance between the outside surfacesof walls 106 a and 106 b. The cell's width is defined by the distancebetween the outside surface of walls 107 a and 107 b. Casing 100 isdesirably coated on its inside surface with a layer of carbon or indiumto improve conductivity. Cell contents comprising anode 150, cathode 110and separator 140 therebetween are supplied through the open end 102. Ina preferred embodiment the anode 150 comprises particulate zinc and thecathode 110 comprises nickel oxyhydroxide. An aqueous solution ofpotassium hydroxide forms a portion of the anode and cathode.

[0074] The cathode 110 may be in the form of a plurality of slabs(disks) 110 a having a hollow central core 110 b through its thickness,shown best in FIG. 5. The cathode slabs 110 a preferably are of overallrectangular shape. The cathode slabs 110 a are inserted into casing 100and stacked vertically one on top of the other along the cell's lengthas shown in FIGS. 2, 3 and 5. Each cathode slab 110 a may be recompactedafter it is inserted into casing 100. Such recompaction assures that theoutside surface of each cathode slab 110 a is in intimate contact withthe inside surface of casing 100. Preferably, the hollow central cores110 b within cathode slabs 110 a are aligned to form one continuouscentral core along the cell's longitudinal axis 190, for receiving anodeslurry 150. Optionally, the cathode slab 110 a closest to the closed end104 of casing 100, can have a bottom surface which abuts and covers theinside surface of closed end 104.

[0075] Cathode slabs 110 a can be die cast or compression molded.Alternatively, cathode 110 can be formed of cathode material which isextruded through a nozzle to form a single continuous cathode 110 havinga hollow core. Cathode 110 can also be formed of a plurality of slabs110 a with hollow core 10 b, wherein each slab is extruded into casing100.

[0076] After cathode 110 is inserted, an electrolyte permeable separator140 is then positioned within central core 110 b of each slab 110 a sothat the outside surface separator 140 abuts the inside surface of thecathode as shown in FIGS. 2, 3, and 5. The inside surface of eachcathode slab 110 a, which defines said hollow central core 110 b, ispreferably a curved surface. Such curved inside surface improves themechanical strength of the slab during transfer and handling and alsoprovides more uniform contact between the separator 140 and the cathode110.

[0077] The central core 10 b of slabs 110 a are aligned to form onecontinuous core as above described. After separator 240 is inserted, thecontinuous core forms the anode cavity 155 for housing anode material150. The anode cavity is within the central core of cathode slab 110 awhich is devoid of cathode material. The anode cavity 155 has anelongated or oblong shape when viewed in plan view (FIG. 4B) upon takinga cross section of a cathode slab 110 a along a plane perpendicular tothe cell's longitudinal axis 190. The cross section of the anode cavity155 defined by boundary perimeter 156 may be elliptical or substantiallyelliptical. Cavity 155 has a long dimension D1 which is greater than itsshort dimension D2. Cavity 155 thus can have a diameter (or width), D1,which is greater along the path of a plane parallel to the wide side(106 a or 106 b) of the cell than its diameter (or width), D2, in thedirection of a plane parallel to a narrow side (107 a or 107 b) of thecell. Thus, by way of nonlimiting example, anode cavity 155 may take theshape of an ellipse or appear to be substantially elliptical when viewedin cross section along a plane perpendicular to the central longitudinalaxis 190. The opposing long boundary edges 158 a and 158 b of the anodecavity when viewed in cross section (FIG. 4B) may be flat orsubstantially flat so that the overall configuration is not a perfectellipse, but nevertheless is of an elongated or oblong shape as shown.

[0078] The cavity 155 has an elongated or oblong shape when viewed incross section (FIGS. 4B, 4C and 4D) obtained by cutting a cathode slab110 a by a plane perpendicular to the cell's central longitudinal axis190. Such elongated shape can be an elongated polygon or a rectangularshape as shown in FIG. 4C. The axes defining the long dimension D1 andshort dimension D2 of cavity 155 defined by boundary perimeter 156 maybe skewed as shown in FIG. 4D. Preferably, the cavity 155 when viewed incross section (FIG. 4B) obtained by cutting a cathode slab 110 a by aplane perpendicular to the cell's central longitudinal axis 190 (FIG.1A) has at least a portion of its boundary perimeter 156 which iscurved. Cavity 155 is desirably of an oblong configuration. In apreferred embodiment substantially all of the boundary perimeter 156defining cavity 155 is curved. In particular it is desirable that atleast the opposing surfaces 157 a and 157 b of cavity 155 closest to theopposing narrow sides 107 a and 107 b of the cell are curved as shownbest in FIG. 4B. Preferably, substantially all of the perimeter 156 ofcavity 155 when viewed in cross section as above described is curved.The opposing long edges 158 a and 158 b are preferably outwardly curved(convex) when viewed from outside casing 100 as shown in FIG. 4B.However, long edges 158 a and 158 b may be flat or substantially flat.Alternatively, long edges 158 a and 158 b may be slightly inwardlycurved (concave) or lightly outwardly curved (convex) or may be ofconvoluted curvature, for example having alternating convex and concavesurfaces. Similarly the short edges 157 a and 157 b may be slightlyinwardly curved (concave) or lightly outwardly curved (convex) or may beof convoluted curvature, for example, having alternating convex andconcave surfaces. In any cross section of a cathode slab 110 a takenalong a plane perpendicular to the cell's longitudinal axis, there is along dimension D1 of cavity 155 representing its maximum length withinthe plane and the short dimension D2 representing its maximum widthwithin said plane. The long diameter (D1) will normally be in thedirection along a plane parallel to a wide side (106 a or 106 b) of thecell and the small diameter (D2) is in the direction along a planeparallel to a narrow side (107 a or 107 b )of the cell as shown in FIG.4B. Cavity 155 when viewed in cross section as above described has atleast some curvature and is characterized in that the ratio of D1/D2 isgreater than 1.0 reflecting that it is of elongated or oblongconfiguration. The shape of the cavity 155 when viewed in a crosssection taken along a plane perpendicular to longitudinal axis 190desirably has a symmetrical oblong configuration having a ratio of D1/D2greater than 1.0. In such case the dimensions D1 and D2 areperpendicular to each other as shown in FIG. 4B. By way of a specificnonlimiting example, the shape of cavity 155 in when viewed in suchcross section is of oblong shape and may be elliptical or substantiallyelliptical.

[0079] Anode 150, is preferably in the form of a gelled zinc slurrycomprising zinc-based particles and aqueous alkaline electrolyte. Theanode slurry 150 is poured into the central core 155 of the cell alongthe cell's longitudinal axis 190. Anode 150 is thus separated fromdirect contact with cathode 110 by separator 140 therebetween.

[0080] After the cell contents are supplied, the cell assembly 12 (FIG.4) is then inserted into the open end 102 to seal the cell and provide anegative terminal 290. The closed end 104 of the casing can function asthe cell's positive terminal. The closed end 104 can be drawn or stampedto provide a protruding positive pip or else a separate end plate 184having a protruding pip 180 can be welded to the closed end 104 of thecasing as shown in FIG. 1A.

[0081] The components comprising a specific embodiment of the end capassembly 12 are shown best in FIG. 4. End cap assembly 12 comprises anelongated anode current collector 160; an insulating sealing member 220;a metal cover 230 which lies over sealing member 220; a metal rivet 240which penetrates partially through insulating sealing member 220; aplastic spacer 250, which insulates rivet 240 from metal cover 230; arubber vent plug 260 seated within a cavity 248 in rivet 240; a vent pipcap 270 over rubber plug 260; a plastic extender seal 280; and anegative terminal plate 290 over plastic extender 280.

[0082] It is herein acknowledged that rubber vent plug 260 as seatedwithin a cavity 248 within a rivet 240, and vent pip cap 270 over rubberplug 260 have been disclosed and used in connection with a commercial7/5-F6 size rectangular rechargeable nickel metal hydride battery ModelNo. GP14M145 made by Gold Peak Batteries, Hong Kong. However, Applicantsof the present patent application herein have determined that the endcap assembly as a whole in said nickel metal hydride rechargeablebattery Model No. GP14M145 causes corrosion and promotes gassing ifapplied to a conventional alkaline primary cell, for example, azinc/MnO₂ alkaline cell. Such corrosion was found to occur between theelongated current collector and the inside surface of the cell housingbecause the widest part of the current collector was very close (lessthan about 0.5 mm) to the cell housing inside surface. It will beappreciated that a wide portion, namely flange 161, of current collector160 is employed in connection with the reseatable vent plug design. Suchwide portion of the current collector (flange 161) is required becausethe current collector is riveted to the underside of insulating sealingmember 220. Thus, flange 161 must be sufficiently wide to fasten base246 of rivet 240 thereto. If the cell 10 is a small size flat cell, forexample a cuboid shaped cell having an overall thickness between about 5and 10 mm, an edge of flange 161 will, therefore, terminate close to aninside surface of casing 100.

[0083] Applicants have modified the subassembly comprising currentcollector 160 and insulating sealing member 220 by redesigning theinsulating sealing member 220 to provide it with a circumventing skirt226. The insulating sealing skirt 226 surrounds the widest part, namelyflange 161 of anode current collector 160. Insulating skirt 161 thusprovides a barrier between the edge of current collector flange 161 andthe inside surface of casing 100. The insulating skirt 161 has beendetermined to reduce the production of corrosive chemicals, typicallymetal containing complexes or compounds, in the space between flange 161and the inside surface of casing 100 during cell discharge. Suchcorrosive chemicals, if produced in quantity, can interfere with cellperformance and promote cell gassing. Also, in the modified designherein described the widest part of the anode current collector 160,namely, flange 161 is between about 0.5 and 2 mm, preferably betweenabout 0.5 and 1.5 mm from the housing inside surface. This incombination with the use of insulating sealing skirt 226 surroundingcurrent collector flange 161 was determined to prevent the production ofany significant amount of corrosive chemicals between current collectorwide portion (flange 161) and the casing 100 inside surface. Suchmodified design of the invention in turn made the reseatable rubber ventplug assembly suitable as a viable vent mechanism for the flat primaryalkaline cell herein described.

[0084] The components of the end cap assembly 12 shown best in FIGS. 4and 5 can be assembled in the following manner: The anode currentcollector 160 comprises an elongated shaft or wire 162 terminating atits bottom end in tip 163 and terminating at its top end in an outwardlyextending integral flange 161, which is preferably at right angles toshaft 162. Thus when the current collector 160 is inserted into anode150, the edge of outwardly extending flange 161 can be closer to theinside surface of casing 100 than shaft 162. Insulating sealing member220 has a top panel 227 and opposing open bottom 228. Insulating sealingmember 220 is preferably of nylon 66 or nylon 612, which is durable,resistant to alkaline, and permeable to hydrogen. Alternatively,insulating sealing member 220 may be composed of polypropylene, talcfilled polypropylene, sulfonated polyethylene or other polyamide (nylon)grades, which are durable and hydrogen permeable. Insulating member 220is preferably rectangular so that it can fit snugly within the open end102 of casing 100. The opposing side walls 226 a and opposing end wall226 b extending from top end 227 of insulating member 220 forms adownwardly extending skirt 226 around top panel 227. Skirt 226 definesthe bounds of open bottom 228 of said insulating sealing member 220.There is an aperture 224 through the top panel 227. There is a metalcover 230 which can be a metal plate having an aperture 234therethrough. There is a metal rivet 240 having a head 247 and base 245.Rivet 240 can be of nickel plated steel or stainless steel. Rivet 240has a cavity 248 within head 247. Cavity 248 passes completely throughrivet head 247 and the rivet shaft 245. The flange 161 of currentcollector 160 is inserted into the open bottom 228 of insulating sealingmember 220 so that the flange 161 of the current collector 160 issurrounded and protected by insulating skirt 226 of said sealing member220. As shown in FIG. 4, flange portion 161 of current collector 160 hasan aperture 164 therethrough. The base 246 of rivet 240 can be passedthrough said aperture 164 and riveted to said flange 161 to keep thecurrent collector 160 in electrical contact with said rivet. In suchembodiment insulating skirt 226 provides a barrier between flange 161 ofthe current collector and the inside surface of the cell's casing 100.It has been determined that narrow gaps, for example, less than about0.5 mm, between any surface of the anode current collector 160 and thecell's casing 100 inside surface can provide regions in which corrosiveby-products can occur during conventional alkaline cell discharge. Thisin turn can passivate neighboring regions of the anode current collector160 and promote gassing. The downward extending skirt 226 of insulatingsealing member 220 is intended to surround outwardly extending portionsof the current collector 160 such as integral flange 161, therebyproviding a barrier between the widest portions of the current collector160 and casing 100. This has been determined to resolve the corrosionproblem and reduce gassing. Applicant has modified the design byredesigning the widest part of the current collector preferably byproviding a barrier, namely an insulating skirt 226 surrounding thewidest part, namely flange 161 of anode current collector 160. Theplacement and effect of skirt 226 are described in greater detail in thefollowing paragraphs herein. In Applicant's modified design hereindescribed the widest part of the anode current collector 160, namelyflange 161, is between about 0.5 and 2 mm, preferably between about 0.5and 1.5 mm from the housing inside surface. Also, circumventinginsulating skirt 226 provided a barrier between current collector flange161 and casing 100. These design features were determined to resolve thecorrosion problem and make the reseatable rubber vent plug assemblysuitable as a viable vent mechanism for the flat primary alkaline cellof the invention.

[0085] In forming end cap assembly 12, the flange portion 161 of currentcollector 160 is positioned so that aperture 164 therethrough is alignedwith aperture 224 through top panel 227 of the insulating sealing member220. The metal cover 230 is positioned over the top panel 227 of theinsulating sealing member 220 so that aperture 234 through metal cover230 is aligned with aperture 224. A plastic spacer disk 250 is insertedover metal cover 230 so that the aperture 252 through spacer disk 250 isaligned with aperture 234 of metal cover 230. In the preferredembodiment (FIG. 4), the base 246 of rivet 240 is passed throughaperture 252 of plastic spacer 250 and also through aperture 234 ofmetal cover 230. Base 246 of rivet 240 is also passed through aperture224 of insulating sealing member 220 and aperture 164 of currentcollector flange 161. Plastic spacer 250 insulates rivet 240 from metalcover 230. The base 246 of rivet shaft 245 extends through aperture 224of the insulating sealing member 220 and underlying aperture 164 withinthe top flange portion 161 of anode current collector 160. Base 246 ofthe rivet shaft can be hammered into place against the bottom surface ofcurrent collector flange 161 using an orbital riveter or the like. Thislocks the rivet shaft in place within aperture 224 of the insulatingsealing member 220 and also secures the current collector 160 to therivet shaft 245. This keeps the current collector 160 in permanentelectrical contact with rivet 240 and prevents the rivet shaft 245 frombeing removed or dislodged from aperture 224 of the insulating sealingmember 220. The rivet head 247 is tightly seated over plastic spacer250. This forms a subassembly comprising rivet 240, plastic spacer 250,metal cover 230, insulating sealing member 220 and anode currentcollector 160. The subassembly can be stored until ready for furtherassembly.

[0086] The assembly process is completed by inserting rubber vent plug260 into cavity 248 within the rivet head 247. Plug 260 is preferably ina truncated conical shape and is designed to fit snugly within cavity248 of rivet head 247. Plug 260 is preferably of a compressible,resilient material which is resistant to alkaline electrolyte. Apreferred material for plug 260 is a rubber, preferably a neoprene orEPDM (ethylene-propylene diene terpolymer) rubber or other alkalineresistant compressible rubber. The surface of the plug 240 is preferablycoated with a nonwetting agent such as Teflon (polytetrafluoroethylene),asphalt or a polyamide. A metal vent pip cap 270 is then inserted overplug 260. The vent pip cap 270 is pressed onto plug 260 with forcesufficient to compress the plug by about 0.55 mm. This has beendetermined to provide a seal which can withstand internal gas pressurebuildup of about 200 psig (13.79×10⁵ pascal). Plug 260 compression canbe adjusted so that the seal can withstand internal pressures typicallybetween about 100 and 300 psig (6.895×10⁵ and 20.69×10⁵ pascal gage),desirably between about 100 and 200 psig (6.895×10⁵ and 13.79×10⁵ pascalgage). Higher degree of compression of plug 260 is also possible, ifdesired, to enable the seal to withstand higher pressures, that is,higher than 300 psig (20.69×10⁵ pascal gage). Conversely reducedcompression of plug 260 is possible, if desired, so that the seal ismaintained up to a pressure thresholds at any desired value below 100psig. The base 273 of vent pip cap 270 can have several downwardlyextending segments which fit into indentations or crevices 253 withinthe top surface of plastic spacer 250 as vent cap 270 is pressed ontoplug 260. This is shown best in FIG. 5. After vent pip cap 270 isinserted over plug 260, thereby compressing said plug within rivet headcavity 248, vent cap 270 is welded to rivet head 247. Plug 260 isthereby maintained compressed within rivet head cavity 248. The plasticextender member 280 is placed over the vent cap head 271. The vent caphead 271 protrudes through aperture 282 within plastic extender 280. Aterminal end plate 290 (negative terminal), is then welded to vent caphead 271. Vent cap 270 is thus welded to both end plate 290 and rivet240. Terminal end plate 290 is constructed of a conductive metal havinggood mechanical strength and corrosion resistance such as nickel platedcold rolled steel or stainless steel, preferably, nickel plated lowcarbon steel. Thus, a completed end cap assembly 12 is formed withterminal end plate 290 in permanent electrical contact with currentcollector 163.

[0087] The completed end cap assembly 12 is then inserted into the openend 102 of casing 100. The current collector shaft 162 penetrates intoanode slurry 150. The edge of metal cover 230 is welded, preferably bylaser welding, to the top peripheral edge 104 of the casing. This holdsthe end cap assembly 12 securely in place and seals the open end 102 ofthe casing as shown in FIGS. 1 and 1A. End terminal plate 290 is inelectrical contact with current collector 160 and anode 150, and thusforms the cell's negative terminal for the zinc/NiOOH alkaline cellembodiment described herein. It will be appreciated that the negativeterminal plate 290 is electrically insulated from casing 100 by plasticextender 280. Rivet 240 and anode current collector 160 is electricallyinsulated from casing 100 by plastic spacer 250 and insulating sealingmember 220. As shown in FIGS. 1A, 2 and 3, pip 180 at the opposingclosed end of casing 100 forms the cell's positive terminal. The pip 180can be integrally formed from the closed end 104 of the casing or may bea formed of a separate plate 184, which is separately welded to theclosed end as shown in FIG. 1A. The completed cell is shown in theperspective views of FIGS. 1 and 1A and in cross sectional views ofFIGS. 2 and 3.

[0088] In operation during cell discharge or storage, if the gaspressure within the cell builds up to exceed the design threshold level,plug 260 becomes unseated within rivet head cavity 248. This will allowgas to escape from within the cell interior through rivet head cavity248, then through vent aperture 272 of vent cap 270 and to the externalenvironment. As pressure within the cell is reduced, plug 260 becomesreseated within rivet head cavity 248.

[0089] As an added safety feature the cell can be provided with a secondventing device which functions to supplement the reseatable plug vent260. The supplemental vent may be designed to activate in a catastrophicsituation, for example, if the user inadvertently attempts to rechargeprimary cell 10 for long periods of time using battery chargers designedfor flat rechargeable cells. The cell 10 of the invention is designed tobe a primary (nonrechargeable cell). Despite ample written notice on thecell label that the cell should not be recharged, it is always possiblethat the user will inadvertently attempt to recharge the cell in aconventional flat battery charger, for example, one designed forrecharging a flat nickel metal hydride cell. If the cell is abused inthis manner by attempting to recharge for a long period of time, thereis risk that the internal pressure level could rise abruptly.

[0090] If cell 10 is inadvertently subjected to recharging (although itis intended as a primary non-rechargeable cell), plug vent 260 willunseat thereby releasing pressure as the internal gas pressure reachesthe design threshold, desirably a pressure between about 100 and 300psig (6.895×10⁵ and 20.69×10⁵ pascal gage). Plug 260 will unseat againshould the pressure again build up upon continued charging for longperiods of time. Such process of plug unseating and reseating can berepeated many times resulting in a pulsed release of gas pressure fromthe cell interior. Under such abuse conditions there is the chance thatKOH electrolyte will gradually enter and crystallize within the freespace between the plug 260 and the inside surface of rivet head cavity248 housing plug 260. (Plug 260 is held compressed within rivet headcavity 248 by vent cap 270 which is welded to the rivet head). Theaccumulation of such crystallized KOH reduces the amount of free spacebetween plug 260 and the inside surface of vent cap 270. This can makeit gradually more difficult for the plug 260 to properly unseat as therecharging process is continued, since there may be less free spaceavailable for plug 260 to expand into when the threshold gas pressure isreached. Accumulation of crystallized KOH within a small amount of freespace within rivet head 248 can also impede proper venting of gasthrough vent aperture 272 in vent cap 270.

[0091] Several improvements in design are proposed herein in order toreduce the deleterious effect of such KOH crystalline buildup betweenplug 260 and the inside surface of vent cap 270:

[0092] The venting system of the invention may include primary andsupplemental venting mechanisms. The primary venting mechanism isactivated when gas pressure within the cell builds up to a designpressure threshold level, P1. It is desirable to have a supplementalventing mechanism which activates at a higher pressure level, P2, in theevent that the primary vent mechanism fails to operate properly or ifthe cell is subjected to an abusive situation resulting in a rapid buildup of gas pressure. One such abusive situation could occur, for example,if the cell were subjected inadvertently to charging for an extendedperiod. In one specific embodiment of the invention a reseatable plug240 is employed as a primary venting mechanism, which activates at adesign threshold pressure P1, between about 100 and 300 psig. Asupplemental venting mechanism can be a weak laser weld anywhere withina cut out portion of the casing body 100 or end cap assembly 12. Theweak laser weld is designed to rupture when gas pressure within the cellreaches a higher pressure P2, desirably between about 400 and 800 psig.There may be a strong weld, preferably contiguous with the weak weld.The strong weld is designed to rupture at yet a higher pressure P3,desirably between about 800 and 2500 psig.

[0093] One embodiment of the supplemental venting system can be providedby forming a weak laser weld along a portion of the laser weldedinterface between the metal cover 230 and inside surface of casing 100.(Metal cover 230 is used to close the open end 102 of casing 100.)Preferably, a weak laser weld 310 a can be applied between a majorportion of one of the metal cover long edges 230 a and casing edge 108 aas shown in FIG. 6. The weak laser weld 310 a can be applied so that theweld thickness is such that it will crack or rupture desirably as gaswithin the cell builds up to a pressure of between about 400 and 800psig (2748×10³ and 5515×10³ pascal). The depth of penetration of theweak weld can be adjusted to achieve the desired rupture pressure. Arupture pressure between about 400 and 800 psig (2748×10³ and 5515×10³pascal gage) can be achieved with a depth of penetration of the laserweld between about 2 and 4 mil (0.0508 and 0.102 mm). The weak weld maydesirably run along a major portion of at least one of the long edges230 a of metal cover 230. Preferably, the weak weld begins at a point312 a which is at least about 1 mm from the corner intersecting shortedge 108 c and long edge 108 a of casing 100. Thus, for a flat 7/5-F6cell the length of the weak weld may be at least about 10 mm, typicallyabout 13 mm.

[0094] The remainder of the perimeter interface between the edge ofmetal cover 230 and the casing edge (casing edges 107 c, 107 d, and 106d) (FIG. 6) can be provided with a strong laser weld 310 b designed torupture at a higher pressure level, for example, a gas pressure betweenabout 800 and 2500 psig (5515×10³ and 17235×10³ pascal gage), typicallybetween about 800 and 1600 psig (5515×10³ and 11030×10³ pascal gage).The depth of penetration of the strong weld can be adjusted to achievethe desired rupture pressure. A rupture pressure between about 800 and2500 psig (5515×10³ and 17235×10³ pascal gage) can be achieved with adepth of penetration of the laser weld between about 5 and 7 mil (0.127and 0.178 mm). The depth of penetration of the strong weld can be morefinely adjusted allowing the weld to rupture at a desired pressurelevel, for example, between about 1300 and 1600 psig (8962×10³ and11030×10³ pascal gage).

[0095] Although the edge of metal plate 230 is shown welded to theinside surface of metal casing 100 as shown in FIG. 6, there can bealternative embodiments wherein the edge of metal plate 230 is welded toan edge of metal casing instead of the inside surface of casing 100. Onesuch alternative embodiment is shown in FIG. 6A. In such embodiment(FIG. 6A) the casing edge 108 defined by opposing long edges 108 a and108 b and opposing short edges 108 c and 108 d are crimped so that suchedges lie in about the same plane as metal plate 230. Thus, the edges230 a and 230 b of metal plate 230 can be laser welded directly to thecrimped casing edge 180. A strong laser weld 310 b can be appliedbetween plate edge 230 b and casing edges 108 b, 108 c, and 108 d; aweak laser weld 310 a can be applied between plate edge 230 a and casingedge 108 a.

[0096] In yet other embodiments there can be cut out portions within thebody of casing 100. The cut out portion may be of varying shape. Forexample, the cut out portion may be polygonal (FIG. 7) or may have atleast a portion of its boundary curved (FIG. 7A). A metal plate, e.g.plate 400 (FIG. 7) or plate 500 (FIG. 7A) may by inserted into such cutout portions. The edges of metal plate 400 (FIG. 7) or metal plate 500(FIG. 7A) may be laser welded to the casing to close the cut outportion. The welds may desirably be in the form of a strong laser weld410 b and a contiguous weak laser weld 410 a as shown in the embodimentof FIG. 7. The welds may in the form of a strong laser weld 510 b andpreferably a contiguous weak laser weld 510 a as shown in the embodimentof FIG. 7A.

[0097] The strong and weak welds can be achieved using different typesof lasers operating within a range of peak power output. The followingare nonlimiting examples of a weak and strong welds produced using aNd:Yag laser. The weak weld as above described will crack or rupture asabove described when gas pressure within the cell reaches a thresholdpressure of between about 400 and 800 psig (2758×10³ and 5515×10³ pascalgage). The strong weld will rupture when the gas pressure within thecell reaches a threshold pressure of between about 800 and 2500 psig(5515×10³ and 17235×10³ pascal gage), typically between about 800 and1600 psig (5515×10³ and 11030×10³ pascal gage). The weak weld inparticular provides a supplemental venting system allowing gas to escapefrom within the cell should the cell be abused as above described.

[0098] Alternatively, the supplemental venting mechanism may be in theform of a grooved vent, that is, one or more grooves on the surface ofcasing 100 which results in an underlying thinned material region. Thedepth of the groove and thickness of the underlying thinned materialregion can be adjusted so that the thinned region ruptures when gaspressure within the cell rises to a pressure P2 greater than P1. Inanother embodiment the reseatable plug can be eliminated and a laserweld can be used as the primary vent mechanism activating at a pressureP1. In such embodiment the supplemental venting mechanism may be athinned material region underlying a groove on the casing surface. Thethinned material may be designed to rupture at a higher pressure levelP2.

[0099] In yet another embodiment there can be a plurality of grooves onthe casing surface. One groove may have underlying thinned regions ofsmall thickness allowing it to rupture as gas within the cell builds upto the design pressure level, P1. A second groove on the casing surface,which may be contiguous with or spaced apart from the first groove, mayhave an underlying thinned region designed to rupture if gas pressurewithin the cell builds up to a higher pressure, P2.

[0100] When the casing 100 is of steel, for example, nickel plated coldrolled steel or stainless steel, a groove such as groove 600 a (FIG. 8)can be formed on the casing surface so that the thinned materialunderlying the groove will rupture when gas pressure within the cellreaches a pressure of between about 400 and 800 psig (2758×10³ and5515×10³ pascal gage). The non grooved portions of the casing wall maytypically have an average wall thickness of between about 0.3 and 0.50mm, desirably between about 0.3 and 0.45 mm. In order to achieve rupturepressures between about 400 and 800 psig (2758×10³ and 5515×10³ pascalgage), the groove is formed so that the underlying thinned material hasa thickness typically of between about 0.07 and 0.08 mm. In a specificnonlimiting example, if the groove is formed so that the underlyingthinned material has a thickness of about 0.074 mm, such underlyingmaterial will rupture when the gas pressure within the cell reaches alevel of about 435 psig (2999×10³ pascal gage).

[0101] A groove 600 which may, for example, be in the form of a groove600 a or 600 b (FIGS. 8-8A) on the casing surface may be made bystamping the surface of the casing with a die, preferably a die having acutting knife edge. A mandrel is held against the inside surface of thecasing as the stamping die punches into the casing outside surface. Forgrooves formed with such stamping or cutting die, the thickness of theunderlying thinned region 610 (FIG. 9) primarily determines the pressureat which the thinned region will rupture. The groove cut may desirablybe V-shaped (FIG. 9), which is obtained preferably with a stamping diehaving a knife edge. The V shaped groove desirably has an acute angle,α, of about 40 degrees. The groove 600 can be made by other methods, forexample, by chemical etching.

[0102] The thinned material underlying groove 600 is preferably the sameas the casing material, typically of nickel-plated cold-rolled steel.The groove 600 may have boundaries which are straight or curved or mayhave a combination of straight and curved portions. The groove 600 mayhave boundaries which are rectangular, polygonal or oblong. The groovemay have at least a portion of their boundaries which are convoluted,that is partially convex and partially concave. The groove boundary maybe closed or open. In a preferred embodiment herein the groove can be astraight or substantially straight, preferably parallel to a wide edge108 a of the casing (FIG. 8-8A). For example, in a 7/5-F6 sizerectangular cell, the groove 600 a (FIGS. 8 and 8A) can desirably belocated parallel to casing wide edge 108 a and about 10 mm therefrom andmay have a length of about 8 mm.

[0103] There may be a second groove 600 b spaced apart from a firstgroove 600 a as shown in FIG. 8A. In a specific non limiting embodimentthe second groove 600 b may be parallel to groove 600 a and about 10 mmform the positive terminal 180 (closed end). Groove 600 b may have alength of about 8 mm. In such case the underlying material thickness ofeach groove 600 a and 600 b may be different so that the underlyingmaterial in each ruptures when gas pressure within the cell reachesdifferent pressure levels. For example, the thinned material underlyinggroove a 600 a can be designed to rupture when gas pressure within thecell reaches a design burst pressure of between about 250 and 800 psig.To achieve such range in burst pressure between about 250 and 800 psig(1724×10³ and 5515×10³ pascal gage), the thinned material underlyinggroove 600 a has a thickness between about 0.04 and 0.15 mm.Alternatively, the thinned material underlying groove 600 a can bedesigned to rupture when gas pressure within the cell reaches a designburst pressure of between about 400 and 800 psig (2758 ×10³ and 5515×10³pascal gage). To achieve such burst pressure between 400 and 800 psigthe thinned material underlying the groove 600 a has a thickness betweenabout 0.07 and 0.15 mm. The thinned material underlying second groove600 b can be designed to rupture when gas pressure within the cellruptures in a catastrophic situation in the event that the cell bemisused and gas pressure within the cell rapidly rises to a level ofbetween about 800 and 1600 psig (5515×10³ and 11030×10³ pascal gage). Inorder to achieve rupture at pressure levels between about 800 and 1600psig the thinned material 610 underlying groove 600 b, typically has athickness of between about 0.15 and 0.35 mm.

EXAMPLE 1 Weak Weld Using Nd:Yag Laser

[0104] The weak weld used to weld metal cover 230 to casing 100 alongcasing edge 106 c (FIG. 6 or 6A) can be produced by employing an Nd:Yaglaser. The laser is operated at a frequency of about 100 Hertz. The peakpower output per pulse is about 35 Watts. The average power output is0.5 Killiwatts. The pulse width (cycle time between peak power) is about0.7 millisecond. The laser feed rate (rate of movement of the laseralong weld path) is about 3 inches per minute. A uniform weld along longedge 106 c of the casing is produced thereby welding abutting long edge230 a of metal cover 230 thereto. The weld had a uniform depth ofpenetration between about of 2 and 4 mils (0.0508 and 0.102 mm),typically about 3 mil (0.0762 mm). The weld cracks when pressure withinthe cell reaches a level of between about 400 and 800 psig (2757×10³ and5515×10³ pascal gage) and thereby functions as a supplemental ventshould the operation of primary vent (plug 260) become compromised.

EXAMPLE 2 Strong Weld Using Nd:Yag Laser

[0105] The strong weld used to weld metal cover 230 to casing 100 alongcasing edges 106 d, 107 c and 107 d (FIG. 6 or 6A) can be produced byemploying an Nd:Yag laser. The strong weld used to weld metal cover 230to casing 100 along casing edge 106 d, 107 c and 107 d can be producedby employing an Nd:Yag laser. The laser is operated at a frequency ofabout 12 Hertz. The peak power output per pulse is about 46 Watts. Theaverage power output is 0.65 Killiwatts. The pulse width (cycle timebetween peak power) is about 5.9 millisecond. The laser feed rate (rateof movement of the laser along weld path) is about 2 inches per minute.A uniform weld along long edges 106 d, 107 c, and 107 d of the casing isproduced thereby welding abutting edges of metal cover 230 thereto asshown in FIG. 6. The weld had a uniform depth of penetration of betweenabout 5 and 7 mils (0.127 and 0.178 mm), typically about 6 mil 0.152mm). The weld cracks when pressure within the cell reaches a level ofbetween about 800 and 2500 psig (5515×10³ and 17235×10³ pascal gage),typically between about 800 and 1600 psig (5515×10³ and 11030×10³pascal).

[0106] In commercial production the above described Nd:Yag laser couldbe operated at higher peak power of about 125 Watts to produce the weakweld and a peak power of about 150 Watts to produce the strong weld.Such operation at higher peak power allows the laser to be moved alongthe weld path (feed rate) at higher speed.

[0107] Additionally in order to reduce the deleterious effect of any KOHcrystalline buildup between plug 260 and the inside surface of vent cap270, which may possibly occur if the cell is inadvertently subjected tocharging, the plug 260 may be modified so that it occupies less spacewithin the cavity in rivet head 247 housing plug 260. A specificembodiment of such improved design is shown as plug 260 in FIGS. 2A, 3A,and 4A. In the improved design of plug 260 shown in FIG. 4 the plugoccupies less space within rivet head cavity 248 than the frustum designfor the plug shown in FIGS. 2, 3 and 4. After the plug 260 of compressedinto the rivet head cavity 248 the amount of free space within cavity248 housing said plug is greater than about 10 percent, desirablybetween about 10 and 40 percent. By contrast the amount of free spacewithin the rivet cavity of plug 260 shown in the frustum embodimentshown in FIGS. 2, 3, and 4 is typically less than about 10 percent.

[0108] It has been determined that the greater amount of free space,typically between about 10 and 40 percent within rivet head cavity 248,which is achieved with the modified plug design (FIG. 4A) assures moreeffective operation of the plug in an abuse situation, for example, ifthe cell is inadvertently subjected to charging as above described. Suchmodified plug 260 (FIG. 4A) desirably unseats as gas pressure within thecell reaches the design threshold level, e.g. between about 100 and 300psig. The greater amount of free space within rivet head cavity 248accommodates any gradual KOH crystalline buildup therein, which mayoccur if the cell is abused by inadvertent charging. There isnevertheless enough free space remaining within rivet head cavity 248 toallow the plug 260 to operate effectively in unseating itself to allowgas pressure to release as pressure within the cell builds up to thedesign threshold level.

[0109] The shape of plug 260 can be altered as shown in FIG. 4A toachieve the greater amount of free space within rivet head cavity 248.In the modified design (FIG. 4A) plug 260 has a cylindrical base 262 andintegrally formed cylindrical body 261 of smaller diameter extendingtherefrom. The ratio of diameter of body 261 to the diameter of base 262can be adjusted as required to achieve the desired greater amount offree space within rivet head cavity 248, typically between about 10 and40 percent, after modified plug 260 (FIG. 4A) has been compressed withinthe cavity. The cylindrical shape of body 261 withstands well thepressure of compressing plug 260 within rivet head cavity 248. Modifiedplug 260 (FIG. 4A) is compressed within rivet head cavity 248 byapplying a force to the top surface 263 of the plug and then weldingvent cap 270 to the rivet head 247. This keeps plug 260 tightly seatedin compressed state within rivet head cavity 248. In the compressedstate the body 261 of plug 260 assumes a bulbous configuration as shownin FIGS. 1A and 2A. When gas pressure within the cell builds up to athreshold level, desirably between about 100 and 300 psig, the plugunseats itself thereby letting gas from within the cell to escapethrough vent apertures 272 within vent cap 270. Preferably there are atleast two vent apertures 272 within vent cap 270 to assure that therewill be a clear path through which gas can escape from vent cap 270 asplug 270 unseats. Plug 260 is desirably of an elastomeric, preferablyrubber material which is sufficiently compressible and resilient, yetresists chemical attack or physical degradation upon contact withalkaline electrolyte. A preferred rubber material for plug 260 isvulcanized EPDM rubber, desirably having a Durometer hardness betweenabout 80 and 85.

[0110] Although the modified configuration for plug 260 as shown in FIG.4A is preferred, it will be appreciated that other shapes of the plugcan help achieve the desired increase in free space within rivet headcavity 248 which houses the plug. For example, the body 261 of the plugcould be slightly sloped instead of cylindrical as shown in FIG. 4A.Also the width of rivet head cavity can be enlarged in the direction ofwide side of the cell (FIG. 2A). In such case the rivet head cavity 248will be elongated along one axis, that is, in the direction of the wideside of the cell (FIG. 2A). However, the symmetrical (circular) rivethead cavity 248, as presently shown in the figures, is preferred.

[0111] It is not intended to restrict the invention to any particularsize rectangular cell. However, by way of particular example, thealkaline cell 100 can be is a small sized rectangular (cuboid),typically having a thickness between about 5 and 10 mm, particularly athickness between about 5 and 7 mm as measured by the outside surface ofthe casing in the direction of the cell thickness. The cell width maytypically be between about 12 and 30 mm and the cell length maytypically be between about 40 and 80 mm. In particular the alkaline cell10 of the invention can be used as a replacement for same sizedrechargeable nickel metal hydride cells, for example, standard 7/5-F6size rectangular cells. An 7/5-F6 size cell has thickness of 6.1 mm,width of 17.3 mm, and length of about 67.3 mm.

[0112] Chemical Composition of a Representative Cell

[0113] The following description of cell composition regarding thechemical composition of anode 150, cathode 110, and separator 140 isapplicable to the flat rectangular alkaline cells disclosed in the abovedescribed embodiments.

[0114] In the above described cell 10, the cathode 110 comprises nickeloxyhydroxide (NiOOH), and an anode 150 comprises zinc, an ion-permeableseparator, and alkaline electrolyte. The anode material 150 can be inthe form of a gelled mixture containing a mercury-free (zero-addedmercury) zinc alloy powder. The anode active material can include forexample, zinc-based particles admixed with conventional gelling agents,such as sodium carboxymethyl cellulose or the sodium salt of an acrylicacid copolymer, and alkaline electrolyte. The alkaline electrolytecomprises a conventional mixture of potassium hydroxide and zinc oxidedissolved in water.

[0115] The cathode 110 desirably has the following composition:

[0116] The cathode preferably can include an active cathode materialcomprising nickel oxyhydroxide (NiOOH), an electrically conductiveadditive that can include conductive carbon particles, preferablyoxidation-resistant graphitic carbon particles, and alkalineelectrolyte. Generally, the cathode can include, for example, between 60and 97 wt. %, between 80 and 95 wt. %, or between 85 and 90 wt. % of thenickel oxyhydroxide. The active cathode material also can comprise anadmixture of nickel oxyhydroxide and manganese dioxide (e.g., EMD) asdisclosed for example in U.S. Pat. No. 6,566,009. Optionally, thecathode also can include an oxidizing additive, a polymeric binder, orcombinations thereof. The oxidizing additive can be more readily reducedthan the active cathode material, thereby serving as a sacrificialadditive. Examples of oxidizing additives include sodium hypochlorite,sodium peroxydisulfate, potassium peroxydisulfate, barium permanganate,barium ferrate or silver oxide. The presence of an oxidizing additivecan serve to stabilize the nickel oxyhydroxide thereby improving thestorage characteristics of the cell.

[0117] The basic electrochemical discharge reaction at the cathodeinvolves reduction of nickel oxyhydroxide according to the followingrepresentative reaction shown in Eq. 1. However, it will be appreciatedthat other secondary reactions (e.g., self-discharge) are possible aswell.

NiOOH+H₂0+1e ³¹ =Ni(OH)₂+OH⁻  (Eq. 1)

[0118] The nickel oxyhydroxide can include one or more nickel oxides.The nickel oxyhydroxide can be selected from beta-nickel oxyhydroxide,cobalt oxyhydroxide-coated beta-nickel oxyhydroxide, gamma-nickeloxyhydroxide, cobalt oxyhydroxide-coated gamma-nickel oxyhydroxide, asolid solution or physical mixture of beta-nickel oxyhydroxide andgamma-nickel oxyhydroxide, a solid solution or physical mixture ofcobalt oxyhydroxide-coated beta-nickel oxyhydroxide and cobaltoxyhydroxide-coated gamma-nickel oxyhydroxide. The nickel oxyhydroxidecan be a powder including particles that are nominally spherical,spheroidal, or ellipsoidal in shape. The average particle size of thenickel oxyhydroxide powder can be between 1 and 100 microns, 2 and 50microns or 5 and 10 microns. The nickel oxyhydroxide also can include atleast one bulk dopant. The bulk dopant can be selected from aluminum,manganese, cobalt, zinc, gallium, indium, or bismuth. The bulk dopantcan be present at a relative weight percentage of less than about 10%,less than about 5% or less than about 2%. The dopant can serve toincrease the conductivity of the nickel oxyhydroxide as well as todecrease the open circuit voltage (OCV) of the cell thereby decreasingoxidation of electrolyte during storage.

[0119] Cobalt oxyhydroxide can be applied to the surface of the nickeloxyhydroxide particles so as to cover at least 60% of their surface,desirably at least 70%, preferably at least 80%, more preferably atleast 90% of the surface. The cobalt oxyhydroxide-coated nickeloxyhydroxide can be formed from nickel hydroxide coated with between 2%and 15%, between 3% and 10%, between 4% and 8% or between 4% and 5%cobalt hydroxide by weight. The cobalt oxyhydroxide coating can serve toenhance inter-particle electrical contact between nickel oxyhydroxideparticles thereby improving bulk conductivity of the cathode. Further,the cobalt oxyhydroxide coating can serve to improve retention ofcapacity after storage for extended periods of time at a hightemperature, for example, 60° C. Optionally, the cobalt oxyhydroxidecoating can include at least one dopant selected from magnesium,calcium, strontium, barium, scandium, yttrium, lanthanum, rare earthelements, titanium, zirconium, hafnium, chromium, manganese, nickel,copper, silver, zinc, cadmium, aluminum, gallium, indium, bismuth orcombinations thereof.

[0120] Suitable commercial nickel oxyhydroxide and cobalt-coated nickeloxyhydroxide can be obtained from Kansai Catalyst Co. (Osaka, Japan),Tanaka Chemical Co. (Fukui, Japan), H.C. Starck GmbH & Co. (Goslar,Germany), Umicore-Canada Inc., (Sherwood Park, Alberta, Canada) or OMGroup Inc. (Westlake, Ohio).

[0121] The electrically conductive additive can be included in anadmixture with nickel oxyhydroxide to enhance bulk electricalconductivity of the cathode. Examples of suitable electricallyconductive additives include carbon particles, nickel powder, cobaltpowder, cobalt oxide, cobalt oxyhydroxide, carbon fibers, carbonnanofibers, fullerenes or combinations thereof. Carbon nanofibers aredescribed, for example, in commonly assigned U.S. Ser. No. 09/658,042,filed Sep. 7, 2000 and U.S. Ser. No. 09/829,709, filed Apr. 10, 2001.More particularly, the cathode can include between 2 and 20 wt. % orbetween 5 and 15 wt. % or between 6 and 8 wt. % of conductive carbonparticles. Conductive carbon particles can include graphitized carbon,carbon black, petroleum coke or acetylene black. Preferably, theconductive carbon is a graphitized carbon. Graphitized carbon caninclude natural graphite, synthetic graphite, expanded graphite,graphitized carbon black, carbon nanofibers, fullerenes or mixturesthereof. Graphitized carbon particles can have a wide variety of shapesincluding substantially spherical, elongated or needle-like having onedimension substantially longer than the others, flake-like having twodimensions elongated relative to a third or fibrous and thread-like.Generally, both natural and synthetic graphite particles can have aflake-like shape. A mixture of graphitized carbon particles can include,for example, from about 10 to 90 percent by weight natural or syntheticgraphite and from 90 to 10 percent by weight expanded graphite. However,the described mixtures are intended to be illustrative and are notintended in any way to restrict this invention.

[0122] In an alkaline cell including nickel oxyhydroxide as the activecathode material, it has been determined to be preferable to use anatural or synthetic graphite that is oxidation-resistant. Nickeloxyhydroxide has an oxidation potential sufficiently high so as topromote oxidation of alkaline electrolytes during storage of the celland can result in the evolution of oxygen gas according to Eq. 2.

2NiOOH+H₂O→2Ni(OH)₂+½O₂  (Eq. 2)

[0123] Oxidation of electrolyte by nickel oxyhydroxide is a well-knownself-discharge process for aqueous nickel electrodes that can decreasedischarge capacity of the cathode. The evolved oxygen gas also canpromote oxidation of the graphite in the cathode. Oxidation of graphitecan decrease bulk conductivity of the cathode as well as form carbondioxide gas according to Eq. 3. The carbon dioxide gas can react furtherwith the electrolyte to form a potassium carbonate solution as shown inEq. 4.

C+O₂→CO₂  (Eq. 3)

CO₂+2KOH→K₂CO₃+H₂O  (Eq. 4)

[0124] An increase in the concentration of carbonate ion in theelectrolyte can decrease ionic conductivity as well as increasepolarization of the zinc anode, thereby decreasing overall cellperformance. The evolved oxygen also can oxidize graphite in theconductive coating applied to the inner surface of cell casing, therebyincreasing contact resistance between the cathode and the cathodecurrent collector. Further, during storage of cells at elevatedtemperatures for extended periods of time, the nickel oxyhydroxide canoxidize graphite in both the cathode and the conductive coatingdirectly. In addition to oxidizing graphite in the cathode, the evolvedoxygen also can diffuse into the anode and react with the zinc-basedparticles of the anode to form zinc oxide, thereby decreasing capacityof the anode. It has been determined as part of the present inventionherein, that the use of an oxidation-resistant graphite as a conductiveadditive in cells also including nickel oxyhydroxide can greatly reducethe undesirable self-discharge processes described hereinabove, therebyminimizing loss of cell capacity, especially after storage at elevatedtemperatures. Desirably, the graphite included in a cathode includingnickel oxyhydroxide can include, for example, from 10 wt. % to 90 wt. %,preferably 100 wt. % of an oxidation-resistant graphite based on thetotal graphite.

[0125] In general, the oxidation-resistance of a graphite is determinedby many contributing factors. For example, it is believed that the rateof graphite oxidation is at least partially related to the specificsurface area of the graphite particles whereby, the smaller the surfacearea, the more oxidation-resistant the graphite. Similarly, oxidationresistance of a graphite can be at least partially related to theaverage particle size as well as the particle size distribution. Becauselarger size particles typically can have lower surface areas, they canbe expected to be more resistant to oxidation. Thus, a graphite having aparticle size distribution with a large percentage of small particleswill be less oxidation-resistant than one with a smaller percentage ofsmall particles. However, the average particle size still must besufficiently small to form an effective conductive network inside thecathode, whereby the graphite particles can be in intimate contact withboth nickel oxyhydroxide particles and other graphite particles. Thus,an oxidation-resistant graphite can have an average particle sizedesirably between about 3 and 30 microns, preferably between about 5 and20 microns. Also, oxidation resistance is believed to be at leastpartially related to the average crystallite size of the graphiteparticles as determined by x-ray diffraction, whereby the larger theaverage crystallite size, the more oxidation-resistant the graphite. Anaverage crystallite size along the a-axis direction of thecrystallographic unit cell, L_(a), of greater than about 2000 angstromsand along the c-axis direction, L_(c), of greater than about 1500angstroms is desirable. Further, it is believed that oxidationresistance also can depend, at least partially, on the relative numberof surface defects or dislocations present in the graphite particles.The relative number of defects can be represented by the ratio of theintegrated area underlying the “D” absorption band centered between 1330and 1360 cm⁻¹to the integrated area underlying the “G” absorption bandcentered between 1570 and 1580 cm⁻¹ in the first order laser Ramanabsorption spectrum. Typically, small size graphite particles havehigher defect levels than larger particles of the same graphite.Desirably, an oxidation-resistant graphite can have relatively lowlevels of defects, for example, corresponding to a defect ratio lessthan 0.15.

[0126] Typically, an oxidation-resistant graphite can be prepared bytreating a high purity natural or synthetic graphite in a non-oxidizingatmosphere at very high temperatures, for example, greater than about2500° C. or about 3000° C. Treating a high purity synthetic graphite ata high graphitization temperature for an extended period of time canproduce graphite having a higher degree of crystallinity, a largeraverage crystallite size, fewer surface defects, lower specific surfacearea, and higher chemical purity (i.e., lower ash content) than thestarting graphite. An oxidation-resistant graphite with a maximum ashcontent of less than about 0.1 wt. % is desirable and less than about0.05 wt. %, more desirable.

[0127] Suitable oxidation-resistant synthetic graphites are availablecommercially under the trade designation “TIMREX® SFG” from TimcalAmerica Co. (Westlake, Ohio). SFG-type graphites suitable for use in anadmixture with nickel oxyhydroxide in the cathode of the cell of theinvention include SFG44, SFG15, SFG10, and SFG6. Preferredoxidation-resistant synthetic graphites include TIMREX SFG10 and SFG15.(The number appearing after the SFG designation refers to the d₉₀particle size which is defined as follows: 90 volume percent of theparticles in a distribution plot of particle size versus volume percenthave a particle size in microns less than the indicated number asdetermined by the laser diffraction method. For example, SFG10 graphitehas a d₉₀ particle size of about 10 microns. It should be noted that themean average particle size typically is much smaller, for example, 7microns for SFG10.) Other suitable synthetic oxidation-resistantgraphites are available from Timcal under the trade designation TIMREX®SLP50 and SLX50.

[0128] A suitable oxidation-resistant, heat-treated natural graphite isavailable, for example, under the trade designation 2939 APH-M fromSuperior Graphite Co.(Chicago, Ill.). The properties and performance incells of this oxidation-resistant purified natural graphite appear to becomparable to those of the above referenced oxidation-resistantsynthetic graphites.

[0129] An oxidation-resistant synthetic graphite also can be dispersedin an organic solvent or water including a film-forming binder, and asurfactant or dispersing aid and applied in a continuous layer to theinside surface of the cell casing. After drying, the coated layer isconductive and can improve electrical contact between the cathode andthe cell casing that serves as the cathode current collector. Such anoxidation-resistant coating also serves to maintain good conductivitybetween a cathode containing nickel oxyhydroxide and the cell casing,especially after storage at elevated temperatures or at ambienttemperature for long periods of time.

[0130] An alkaline cell with a cathode comprising nickel oxyhydroxidecharacteristically has a flat discharge voltage profile that maintainsthe closed circuit voltage during discharge better than conventionalalkaline cells with a cathode comprising manganese dioxide, particularlyat high drain rates. Nickel oxyhydroxide also can be included in anadmixture with manganese dioxide to increase the average closed circuitvoltage of a conventional alkaline cell.

[0131] The Anode 150 has the Following Composition:

[0132] Anode 150 can include between about 60 and 80 wt. %, between 62and 75 wt. %, preferably between about 62 and 72 wt. % of zinc-basedparticles. Preferably, the zinc-based particles include zinc alloyparticles comprising 99.9 wt % zinc containing 200 to 500 ppm indium asalloy and plated material. The term “zinc” as used herein shall beunderstood to include zinc alloy which can comprise a very highconcentration of zinc, for example, at least 99.9 percent by weightzinc. Such a zinc alloy can function electrochemically essentially aspure zinc. The zinc-based particles can have a mean average particlesize between about 1 and 350 microns, between about 1 and 250 micron,and preferably between about 20 and 250 micron. The zinc-based particlescan be predominantly acicular, flake-like or spherical in shape. Thebulk density of zinc-based particles is typically between about 1.75 and2.2 grams zinc per cubic centimeter of anode.

[0133] In general, anode 150 also can include an alkaline electrolyte, agelling agent, and a surfactant. The percentage of electrolyte in theanode is preferably between about 69.2 and 75.5% by volume of the anode.The electrolyte can be an aqueous solution of an alkali metal hydroxide,such as potassium hydroxide, sodium hydroxide, lithium hydroxide ormixtures thereof. The electrolyte can contain between 15 and 60 wt. %,between 20 and 55 wt. % or between 30 and 50 wt. %, for example, 38 wt.% of the alkali metal hydroxide. The electrolyte can contain from 0 to 6wt. % of a dissolved metal oxide, for example, about 2 wt. % zinc oxide.Suitable gelling agents can include a cross-linked polyacrylic acid, ahydrolyzed polyacrylonitrile grafted onto a starch backbone, apolyacrylic acid salt, a carboxy-methylcellulose, acarboxymethylcellulose salt (e.g., sodium carboxymethylcellulose) orcombinations thereof. Examples of a polyacrylic acid include CARBOPOL940 and 934 (available from B.F. Goodrich) and POLYGEL 4P (availablefrom 3V) and a grafted starch material include WATERLOCK A221 and A220(available from Grain Processing Corp.). An example of a polyacrylicacid salt includes ALCOSORB G1 (available from Ciba Specialties). Theanode can include, for example, between 0.05 and 2 wt. % or between 0.1and 1 wt. % of one or more gelling agents. Other representative gellingagents for zinc anodes are disclosed in U.S. Pat. No. 4,563,404.Suitable surfactants include an organic phosphate ester-based surfactantavailable under the tradename GAFAC RA600 (available from Rhône-Poulenc)and a dionyl phenol phosphate ester-based surfactant available under thetradename RM-510 (available from Rhône-Poulenc). The surfactant can beadded to the anode at a level of between 100 and 1000 ppm by weight.Such anode compositions are given only as illustrative examples and arenot intended to restrict the present invention.

[0134] The zinc anode half-cell reactions on cell discharge are given byEq. 5 and Eq. 6:

Zn→Zn⁺²+2e ⁻  (Eq. 5)

Zn⁺²+2OH⁻→ZnO+H₂O   (Eq. 6)

[0135] Anode 150 preferably comprises zinc fines that can be included inan admixture with other zinc-based particles having larger averageparticle size. One convenient way to describe the relative amount ofzinc fines in the total zinc-based particles is the percentage by weightof the zinc-based particles that can pass through a 200 mesh size screen(sieve). Thus, as used herein, “zinc fines” are zinc-based particlessmall enough to pass through a 200 mesh screen. The referenced mesh sizeis a Tyler standard mesh size commonly used in the industry andcorresponds to a U.S. standard sieve having a 0.075 mm square opening.(Tables to convert specific Tyler mesh sizes to square openings inmillimeters are available, for example, in U.S. Standard ScreenSpecification ASTME-11.) Table 1 is an abbreviated conversion table.TABLE 1 Sieve-Square Tyler Opening, mm Standard 0.850  20 mesh 0.250  60mesh 0.150 100 mesh 0.106 150 mesh 0.075 200 mesh 0.063 250 mesh 0.045325 mesh 0.038 400 mesh

[0136] The anode desirably can include at least 10 wt. %, at least 30wt. %, at least 50 wt. % or at least 80 wt %, typically between 35 and75 wt. % of the total zinc-based particles small enough to pass througha 200 mesh size screen. For example, −200 mesh size zinc fines can havea mean average particle size between about 1 and 75 microns, forexample, about 75 microns.

[0137] Even relatively small amounts of zinc fines, for example, about 1wt. %, or about 5 wt. % of the total zinc-based particles small enoughto pass through a 200 mesh size screen, can produce a beneficial effecton overall cell performance. At least 25 wt. %, for example, at least 50wt. % of the zinc-based particles can be of a larger size (e.g.,−20/+200 mesh) such that they can pass through a screen between about 20and 200 mesh size (i.e., a screen having square openings between about0.850 mm and 0.075 mm). For example, when the zinc-based particles inthe anode include about 10 wt. % zinc fines of −200 mesh size in anadmixture with 90 wt. % of larger size zinc-based particles of between−20 and +200 mesh size, the mean average particle size of the totalzinc-based particles can be, for example, about 340 microns. When thezinc-based particles in the anode include 50 wt. % zinc fines of −200mesh size in an admixture with 50 wt. % of larger size zinc-basedparticles of between −20 and +200 mesh size, the mean average particlesize of the total zinc-based particles can be, for example, about 200microns. When 100 wt. % of the zinc-based particles in the anode arezinc fines of −200 mesh size, the mean average particle size of thetotal zinc-based particles can be, for example, about 75 microns.

[0138] It will be appreciated that although the zinc fines preferablycan form a portion of the total zinc-based particles in the anode, thisis not intended to exclude the possibility that a portion of the totalzinc-based particles also can be present in the form of agglomeratedzinc-based particles with or without zinc fines present. Agglomeratedzinc-based particles are disclosed in commonly assigned U.S. Pat. No.6,300,011.

[0139] In another embodiment, at least about 10 wt. %, about 45 wt. % orabout 80 wt. % of the total zinc-based particles can pass through a 325mesh size screen (i.e., 325 mesh corresponds to a screen square openingof about 0.045 mm). The mean average particle size of zinc-basedparticles capable of passing through a 325 mesh size screen cantypically be between about 1 and 35 microns, for example, between about5 and 35 microns or between about 5 and 25 microns.

[0140] Even relatively small amounts, for example, about 5 wt. % orabout 1 wt. % of zinc-based particles that are small enough to passthrough a 325 mesh screen, can produce a beneficial effect on overallcell performance. At least 25 wt. %, for example, at least 50 wt. % ofthe zinc-based particles can be of larger size (e.g., −20/+325) suchthat they can pass through a screen between about −20 and +325 mesh size(i.e., a screen having square openings between about 0.850 mm and 0.045mm). For example, when the total zinc-based particles in the anodeinclude about 10 wt. % zinc fines of −325 mesh size in an admixture withabout 90 wt. % of larger size zinc-based particles of between −20 and+325 mesh size, the mean average particle size of the total zinc-basedparticles can be about 314 microns. When the total zinc-based particlesin the anode include 50 wt. % zinc fines of −325 mesh size in anadmixture with 50 wt. % of larger size zinc-based particles of between−20 and +325 mesh size, the mean average particle size of the totalzinc-based particles can be, for example, about 125 microns. When thetotal zinc-based particles in the anode include 70 wt. % zinc fines of−325 mesh size in an admixture with 30 wt. % of larger size zinc-basedparticles of between −20 and +325 mesh size, the mean average particlesize of the total zinc-based particles can be, for example, about 50microns. When 100 wt. % of the zinc-based particles in the anode arezinc fines of −325 mesh size, the mean average particle size of thetotal zinc-based particles can be, for example, about 35 microns.

[0141] Particle size as reported herein shall be construed as determinedby the more common method employed in the art, namely, by laserdiffraction using the Fraunhofer algorithm for computing the volumedistribution of the particle sizes and the corresponding value for meanaverage. The laser diffraction method is described, for example, by M.Puckhaber and S. Rothele, in “Laser Diffraction—Millennium Link forParticle Size Analysis” (Powder Handling and Processing, Vol. 11, No. 1,Jauary/March 1999). The method determines particle size in terms of amapped spherical equivalent. For example, in the extreme case of anacicular shaped particle, the mapped spherical equivalent can bevisualized as the sphere revolution resulting from rotation of theparticle about its central axis (i.e., the short axis perpendicular toits long axis). The term “average particle size” as used herein and inthe claims, unless otherwise specified, shall be understood to be themean average value based on a plot of distribution of particle sizeversus cumulative volume percent.

[0142] Another, somewhat less accurate, traditional method can be usedto determine the average particle size and size distribution, namely,the sieve method. In this method, the particle size distribution can beobtained by causing the total mixture of particles to pass through aseries of sieves stacked such that the sieve having the largest squareopenings (i.e., smallest mesh size) is positioned at the top of thestack and sieves having progressively smaller square openings (i.e.,larger mesh sizes) are stacked sequentially toward the bottom of thestack. From a graphical plot of particle size, y, versus cumulativevolume percent, x, of the relative volumes of particles retained betweenthe sieves, the mean average particle size can be calculated as theintegral of y dx/100, equal to the area under the plot divided by thebase of 100 volume percent. However, because of better accuracy andwider usage, the mean average particle size reported herein is thatdetermined using the laser diffraction method.

[0143] Preferably, zinc-based particles included in the anode of theflat zinc/NiOOH alkaline cell of the invention can be acicular in shape,defined herein as having a length along a major axis at least two timesa length along a minor axis. The zinc-based particles also can begenerally flake-like in shape, defined herein as having a flakethickness of no more than about 20% of the maximum linear dimension ofthe particle. Inclusion of zinc fines in the anode of the flatzinc/NiOOH alkaline cell of the invention has been determined by theapplicants to improve cell performance at both high and low drain rates.

[0144] Anode 150 typically can have a total mercury content less thanabout 100 parts of mercury per million parts (ppm) of zinc by weight,preferably less than 50 parts of mercury per million parts of zinc byweight. The anode preferably does not contain any added amounts of leadand thus, is essentially lead-free, that is, total lead content is lessthan 30 ppm, desirably less than 15 ppm of the total zinc in the anode.

[0145] In general, because the level of gassing in the zinc/NiOOH cellof the invention is substantially less than that in a zinc/MnO₂ cellhaving the same size and configuration, cell 10 can be balanced in aconventional manner such that the ratio of the theoretical capacity (inmAmp-hr) of nickel oxyhydroxide (based on 292 mAmp-hr per gram NiOOH)divided by the theoretical capacity (in mAmp-hr) of zinc (based on 820mAmp-hr per gram zinc) is about 1, for example, 0.98. In addition, thezinc/NiOOH cell of the invention can be balanced such that either NiOOHor zinc can be in slight excess, for example, the ratio of thetheoretical capacity (in mAmp-hr) of nickel oxyhydroxide divided by thetheoretical capacity (in mAmp-hr) of zinc can be between about 0.95 andabout 1.05.

[0146] Separator 140 can be a conventional ion-permeable separatorconsisting of an inner layer of a nonwoven material of cellulosic andpolyvinylalcohol fibers and an outer layer of cellophane. The non-wovenmaterial can contain from 78 wt. % to 82 wt. % polyvinyl alcohol andfrom 18 wt. % to 22 wt. % cellulosic rayon. Such a material is onlyillustrative and is not intended to restrict this invention.

[0147] It has been determined to be desirable to have a wall thicknessof casing 100 of between about 0.30 and 0.50 mm, typically between about0.30 and 0.45 mm, preferably between about 0.30 and 0.40 mm, moredesirably between about 0.35 and 0.40. Cell 10 is preferably of cuboidshape (FIGS. 1 and 2) having an overall thickness desirably betweenabout 5 and 10 mm.

[0148] Casing 100, is preferably of nickel-plated steel. Casing 100 isdesirably coated on the inside surface with a conductive carbon coating,desirably a graphitic carbon coating. The graphitic carbon coatingpreferably comprises an oxidation-resistant graphite. Such graphiticcarbon coatings can be formed from, for example, an aqueous-basedgraphite dispersion, that can be applied to at least 60%, preferablyfrom about 75 to 90% of the inner surface of the casing and subsequentlydried under ambient conditions. The graphitic carbon coating can serveto maintain good electrical contact between cathode 110 and casing 100and also indirectly reduce the level of gassing by reducing surfacecorrosion on the inner surface of the casing. The metallic cover 230,negative terminal plate 290 and positive terminal plates 180 are alsopreferably of nickel-plated steel. Current collector 160 can be formedfrom a variety of known electrically conductive metals determined to beuseful as current collector materials, for example, brass, tin-platedbrass, bronze, copper or indium-plated brass. Insulating sealing member220 is preferably of nylon 66 or nylon 612.

[0149] Another preferred embodiment of the invention is shown in FIGS.10-17. Although a reusable or reactivatable vent mechanism such asreseatable plug 260 is desirable, it can be eliminated as in theembodiment shown in FIGS. 10-17. It was already indicated hereinabovethat reseatable plug 260 can be eliminated and a weak laser weld can beused as a primary vent mechanism which cracks or ruptures at theactivation pressure of the reseatable plug, namely a pressure P1preferably between about 100 to 300 psig (689×10³ and 2068×10³ pascal).In such case (i.e., with the reseatable plug eliminated), it was statedhereinabove that the supplemental venting mechanism may consist of athinned material region underlying a groove (groove 600 a or 600 b) onthe casing surface. The thinned material may be designed to rupture at ahigher pressure level, P2.

[0150] As shown in the embodiment of FIGS. 10-17, it has been determinedthat one or more groove vents, preferably a single groove vent 600 a or600 b can be used as the primary venting mechanism. In such case thesingle groove vent such as 600 a or 600 b can be cut or scored into thecasing surface so that the underlying thinned region ruptures atpressure P1. Preferably, the single groove vent 600 b is located inproximity to the closed end of the casing in proximity to positiveterminal 180 as shown in FIG. 10A. The groove boundary may be closed oropen. In a preferred embodiment herein the groove can be a straight orsubstantially straight, preferably parallel to a wide edge 108 a of thecasing (FIG. 8-8A). For example, in a 7/5-F6 size rectangular cell,there may be a groove vent 600 b located on the casing wide face 106 a(FIG. 10A). In a preferred embodiment groove vent 600 b comprises theonly groove vent on the casing surface. By way of nonlimiting example,groove 600 b may be a straight groove parallel to the closed end of thecell (FIG. 10A) Preferably the ends of straight groove 600 b may beequidistant from the narrow sides of the casing. In such preferredembodiment groove 600 b may be about 8 mm in length and about 5 to 10 mmfrom the closed end 104 of the casing as shown in FIG. 10A. Although thestraight groove 600 b is desirable, it is not intended to limit suchgroove to such configuration. It will be appreciated that the groovevent 600 b may have other configurations, such as curvilinear shape orpartially straight and partially curvilinear and may form an open orclosed boundary pattern.

[0151] The thinned material underlying groove 600 b (FIG. 10A) can bedesigned to rupture when gas pressure within the cell reaches a designburst pressure of between about 250 and 800 psig (1724×10³ and 5515×10³pascal gage). To achieve such range in burst pressure between about 250and 800 psig (1724×10³ and 5515×10³ pascal gage), the thinned materialunderlying groove 600 b has a thickness between about 0.04 and 0.15 mm.Alternatively, the thinned material underlying groove 600 b can bedesigned to rupture when gas pressure within the cell reaches a designburst pressure of between about 400 and 800 psig (2758×10³ and 5515×10³pascal gage). To achieve such burst pressure between 400 and 800 psig(2758×10³ and 5515×10³ pascal gage) the thinned material underlying thegroove 600 b has a thickness between about 0.07 and 0.15 mm. The widthof groove 600 b (FIG. 10A) at its base (adjacent thinned area 610) maytypically be between about 0.1 and 1 mm, more typically between about0.1 and 0.5 mm. Within such range the rupture pressure is controlledprimarily by the thickness of the underlying thinned region. However, itwill be appreciated that wider grooved or cut areas with underlyingthinned regions on casing 100 are also possible.

[0152] Groove 600 b may be made by stamping the surface of casing 100with a die, preferably a die having a cutting knife edge. A mandrel isheld against the inside surface of the casing as the stamping diepunches into the casing outside surface. Groove 600 b can be cut in a Vshape having equal length sides shown in FIG. 9 or in the V shape havingunequal length sides as shown in FIG. 15. In the former case (FIG. 9)the sides forming the V shaped groove desirably has an acute angle, α,of about 40 degrees and in the latter case (FIG. 15) it has an anglepreferably between about 10 to 30 degrees. In conjunction with groove600 b there may be one or more laser welds securing metal cover 230 tocasing 100. Such welds may be comprise one or both a weak laser weld anda strong laser weld which may be designed to rupture at pressures higherthan the rupture pressure of the thinned region underlying groove 600 b.As earlier described such laser welds may be made using an Nd:Yag laser.The placement of contiguous weak and strong laser welds, for example,weld 310 a and 310 b, used to secure metal cover 230 to casing 100 havebeen shown and described with reference to FIGS. 6 and 6A. In apreferred embodiment there may be just one laser weld, namely, a strongweld 310 b securing the entire circumferential edge of metal cover 230to casing 100 as shown in FIG. 10. The strong weld 310 may be designedto rupture under catastrophic conditions for example, if the cell wereinadvertently subjected to recharging under extremely high current orunder extremely abusive conditions causing gas generation within thecell to rise abruptly to levels between about 800 psig and 2500 psig(5515×10³ and 17235×10³ pascal gage).

[0153] Thus, in a revised preferred cell embodiment shown in FIGS. 10and 13, the single groove 600 b functions as the cell's primary ventingmechanism wherein the thinned material 610 (FIGS. 9 and 15) underlyinggroove 600 b is designed to rupture if gas within the cell rises to alevel between about 250 and 800 psig (1724×10³ and 5515×10³ pascalgage), more typically between about 400 and 800 psig (2758×10³ and5515×10³ pascal gage). And the strong laser weld 310 b securing theedges of metal cover 230 to casing 100 (FIG. 10) is designed to functionas the cell's supplemental venting system. Such strong laser weld 310 bis designed to crack or rupture in the event of a catastrophic situationwhen gas pressure within the cell rises abruptly to levels between about800 psig and 2500 psig. The rupture of laser weld 310 b under suchcatastrophic situations will allow gas to escape therethrough quicklyreducing gas pressure within the cell to nominal levels.

[0154] The cell shown in FIGS. 10 and 10A is shown in the cross sectionview of FIG. 11 taken through a plane parallel to large side 106 a andin cross section view of FIG. 12 taken through a plane parallel tonarrow side 107 a. As shown in FIGS. 11 and 12 the reseatable plug 260incorporated the previous specific embodiment (FIG. 2 and FIG. 4) hasbeen eliminated. It was also possible to eliminate related componentsforming the cell's end cap assembly 12, namely, the elimination of ventpip cap 270 and plastic spacer disk 250 shown in earlier embodiments(FIGS. 2 and 4). The reduction in number of number components formingthe end cap assembly 12 is also reflected in FIG. 13 which is anexploded view of the cell's internal components. Thus, in the revisedembodiment (FIGS. 10-13) rivet 240 can be welded directly to thenegative terminal plate 290 as shown best in FIGS. 11 and 12.

[0155] Welding of the rivet 240 to the negative terminal plate 290 canbe efficiently performed by electrical resistance welding. Rivet 240 ispreferably of brass, desirably brass plated with a layer of tintypically between about 1 to 3 micron thick. The negative terminal plate290 is preferably of nickel plated cold rolled steel. It has beendetermined that the welding can be most efficiently performed and auniform weld readily achieved when the thickness of the terminal plate290 at the weld site is about 4 mil (0.102 mm). It is desirable that theremaining portion of terminal plate 290 be thicker, preferably about 8mil (0.204 mm) for overall strength. Since 8 mil thick cold roll steelplate is commercially available, it was determined that the centralportion 290 a (FIG. 13) of terminal plate 290, that is, the weld areacan be readily be stamped with a mandrel to reduce its thickness of theplate at the weld site to about 4 mil (0.102 mm). The stamping ispreferably done from the top side of terminal plate 290 (FIG. 13) toform stamped area 290 a. Such procedure will leave the thickness of theremaining portion 290 b (FIG. 13) of terminal plate 290 at its original8 mil (0.204 mm) thickness.

[0156] The central portion 290 a (FIG. 13) of terminal plate 290 can bestamped in various configurations, for example, rectangular, circular oroval. In the case of a rectangular or oval stamped area 290 a the longdimension of the stamped area 290 a (in the direction of wide side ofthe casing) should be somewhat greater than the rivet head 247 diameterto allow for placement of the welding electrodes (not shown). Forexample, with a 3 mm diameter rivet head 247, the long dimension of thestamped area 209 a may be about 4 mm and the short dimension of thestamped area 290 a may be between about 3 to 4 mm. If the stamped areais circular, its diameter may be between about 3 to 4 mm, if the rivethead 247 diameter is about 3 mm.

[0157] In assembling the flat alkaline cell 10 of revised embodiment(FIGS. 10-13), anode material 150 comprising zinc fines and cathodematerial comprising nickel oxyhydroxide with separator 140 therebetweenis inserted into cell casing 100 through open end 102. End cap assembly12 (FIG. 13) is formed by inserting metal cover 230 over insulating sealmember 220 so that the protruding head 221 of the seal member isinserted into aperture 234 of the metal cover 230. The rivet shaft 240is then inserted through aperture 234 in metal cover plate 230 (FIG.13). The rivet shaft 240 is insulated from metal cover 230 by protrudinghead 221 of the insulating seal member 220. The top flange portion 161of anode current collector 160, preferably of brass, is pushed upagainst the bottom surface of sealing member 220 and fastened to thelower portion of rivet 240, preferably of brass or tin-plated brass byconventional riveting technique, thus electrically connecting currentcollector 160 to rivet 240. There is an aperture 164 in currentcollector flange 161 and also an opening 249 a in the lower portion ofrivet 240 to make it easier to fasten anode current collector 160 torivet 240. (The upper portion 249 b of rivet 240 is desirably solid asshown in FIGS. 11 and 12.) Upon fastening the current collector 160 torivet 240 sealing member 220 becomes wedged between current collectorflange 161 and the bottom surface of metal cover 230.

[0158] As shown best in FIGS. 10 and 11, metal cover 230 is welded by astrong laser weld (rupture pressure preferably 800 to 2500 psig) to theinside surface of casing 100. The central portion of metal cover 230 isindented or depressed forming well region 235 (FIG. 11). A sealingmaterial 236, preferably an asphalt sealant available under the tradedesignation KORITE asphalt sealant (Customer Products) is applied to thedepressed well region 235 on the surface of metal cover 230 (FIGS. 11and 13). KORITE asphalt sealant is a non-hardening asphalt comprising amixture of about 55 to 70 wt. % asphalt and 30 to 45% asphalt. Theviscosity of the sealant can be adjusted by adjusting theasphalt/solvent ratio and temperature of the sealant as it is beingapplied. Preferably, the well region 235 within cover plate 230 andrivet head 247 are heated before the asphalt sealant is poured into thewell region 235. Desirably, the asphalt sealant is at a viscosity ofabout 1000 centipoise as it is being poured into well region 235. Theasphalt fills depressed region 235 and provides additional sealingprotection between rivet 240 and sealing member 220. The subassemblycomprising metal cover 230 rivet 240 and insulating seal member 220 isthen inserted into the open end 102 of the casing so that the currentcollector shaft 162 penetrates into the anode material 150 (FIGS. 11 and12). After the circumferential edge of metal cover 230 is laser weldedto casing 100, a plastic extender seal 280 (FIG. 13) can be insertedover rivet 240. This is done by inserting rivet head 247 throughaperture 282 of the plastic extender 280. A flat negative metal plate290 is then placed onto the top surface of plastic extender 280 so thatit contacts the protruding rivet head 247. The negative plate 290 isthen welded to rivet heat 247 to complete construction of end capassembly 12. The completed end cap assembly 12 in cell 10 is shown inFIGS. 11 and 12.

[0159] In laser welding the edge of metal cover 230 to casing 100 it ispreferred to apply a heat conductive medium, desirably a liquid or metalto metal cover 230. The heat conductive medium is preferably a liquidwhich can be conveniently placed within the well or trough region 235 onthe surface of metal cover 230. Such heat conductive medium can absorbsa substantial portion of the heat generated during the laser welding.Sufficient heat can be absorbed in this manner to prevent metal cover230 from reaching a temperature higher than about 100° C. during thewelding when employing a Nd:Yag (or equivalent) laser. Alternatively,the metal cover can be precooled to a temperature less than ambient roomtemperature (21° C.), preferably to a temperature approaching thefreezing point of water and even lower. Such precooling can also help tokeep metal cover 230 from exceeding 100° C. during the weldingoperation. Additionally, the metal cover 230 can be precooled and heatconductive medium also applied to the metal cover 230.

[0160] Plastic extender seal 280 is press fitted into casing 100 abovemetal cover 230. If the metal cover 230 is not kept sufficiently coolduring laser welding, seal 280 could possibly overheat to a pointadversely affecting its physical properties. This could result in a lossin radial compression, that is, a loosing of the tight fit between seal280 and casing 100, in turn providing a path to the casing exterior forelectrolyte which may have leaked into the region between metal cover230 and metal cover 280. Thus, it is recommended to keep metal cover 230sufficiently cool, or apply a heat conductive medium to the metal coverto assure that sufficient heat is absorbed during the laser weldingoperation. A convenient way of accomplishing this is to simply add asmall amount of cool deionized water, e.g at about 5 to 10° C., to thetrough region 235 of metal cover 230. This keeps the temperature ofmetal cover 230 below about 100° C., in turn allowing seal 280 (FIGS. 11and 12) to stay within a favorable working temperature. (After the laserwelding operation sealant, e.g. asphalt sealant, can be added to wellarea 235 in the manner earlier described.) Alternatively, a metal block(optionally pre-cooled) can be applied in contact with metal cover 230.The metal block, typically of steel, can absorb sufficient heat duringwelding operation to assure that metal cover 230 does not reachtemperatures above about 100° C.

[0161] Although water can be added to well area 235 to keep metal cover230 cool during the laser operation, it will be appreciated that othersuitable coolants can be used. For example, instead of pure water, acool aqueous solution of polyvinylalcohol and potassium hydroxide may beused. The polyvinylalcohol will coat out on the top surface of metalcover 230 thereby providing additional sealing protection between metalcover 230 and seal 280. Such solution can also be applied directly tothe underside of plastic seal 280 (FIGS. 11 and 12) thereby providingadditional sealing protection to the undersurface of seal 280.

[0162] Alternatively, a gelling agent can be added to the aqueouscooling solution supplied to well 235. The term gelling agent isintended to include conventional gelling agents as well assuperabsorbents. Such gelling agents include polyacrylic acid,polyacrylates, the sodium salt of an acrylic acid copolymer,carboxymethyl cellulose, sodium carboxymethyl cellulose, starch graftcopolymers and the like. The gelling agent will coat the surface ofmetal cover 230 to provide additional sealing protection between metalcover 230 and seal 280. The gelling agent can be applied as a powder tothe underside of plastic seal 280 using compression rollers and the like(FIGS. 11 and 12) to thereby coat seal 280 with gelling agent andprovide additional sealing protection.

[0163] In the embodiment shown in FIG. 17 employing a paper washer 295instead of plastic seal 280, the paper washer can be pre saturated withan aqueous solution comprising a gelling agent or with an aqueoussolution of polyvinylalcohol and potassium hydroxide. Such prewetting ofpaper washer 295 provides additional sealing protection shouldelectrolyte leak into the region between metal cover 230 and terminalplate 290 (FIG. 17. Additionally, a separate layer comprising gellingagent powder can be pressed onto the underside of washer 295, usingcompression rollers and the like or dispersed within the fibrous networkcomprising washer 295, as described in U.S. Pat. No. 4,999,264.

[0164] A separate protruding metal member 180 can be welded to opposingclosed end 104 of the cell. In the completed cell 10 (FIGS. 10-13) metalmember 180 is in electrical contact with the cell casing and thus formsthe cell's positive terminal. End plate 290 is in electrical contactwith rivet 240 which in turn in electrical contact with anode currentcollector 160 and thus forms the cell's negative terminal.

[0165] Although the invention is described in detail in terms of thespecific embodiments, it will be appreciated that variations in designof specific components are possible and are intended to be within theconcept of the invention. For example, the current collector 160 (FIGS.4 and 13) is shown as an elongated member in the shape of a nailterminating at its top end in a horizontally extending flange 161 withan aperture 164 therethrough. This design is attractive, since it makesit easy to fasten the top flange 161 of current collector 160 to thebottom surface 249 (FIG. 11) of rivet 240. This may be accomplished byconventional riveting technique wherein the bottom surface 249 of therivet is inserted into the flange aperture 164 and then crimped tosecure the current collector flange 161 to rivet 240. In such design thecurrent collector shaft is offset, that is does not lie along the cell'scentral longitudinal axis. Such offset does not appear to significantlyinterfere with efficient discharge of the anode or with the overallperformance of the cell. The current collector shaft 162 is shown as astraight nail in the embodiment of FIG. 4 and as straight nail having acurved surface in the embodiment shown in FIG. 13. In either case thenail is offset from the cell's central longitudinal axis 190, as shownfor example, in FIG. 11.

[0166] It will be appreciated that other current collector designs arepossible. For example, rivet 240 may be a solid member having a headportion 247 (FIG. 11) with an integrally formed elongated shaft portion(not shown) in the shape of a nail extending downwardly from head 247and at least partially penetrating into anode 150. In such case thecurrent collector shaft can be oriented so that it is centrally locatedalong the cell's central longitudinal axis 190. The head portion 247 ofsuch rivet can still be welded to the terminal end plate 290 in themanner above described.

[0167] Another specific embodiment of the flat cell of the invention isshown in FIG. 17. In this embodiment a paper washer 295, for example ofKraft paper, can be employed between metal cover 290 and the negativeterminal plate 290 instead of plastic extender 280. The Kraft paperwasher is durable and provides the necessary electrical insulationbetween metal cover 230 and terminal plate 290. Such design reduces theheight of rivet head 247 thereby compacting the total height of the endcap assembly 12. It also has the advantage of eliminating the need tomold a plastic extender 280. This in turn results in more availablevolume for anode/cathode active material within the cell interior. (Theother components of the cell shown in FIG. 17 are essentially the sameas those shown and described with respect to the cell shown in FIGS.10-12.) It will be appreciated that the rivet 240 shown in FIG. 17 withseparate current collector 160 fastened thereto may be replaced with asingle rivet with integral elongated current collector, e.g. in theshape of a nail, extending therefrom and penetrating at least a portionof anode 150. Such integral current collector may be centrally locatedalong the cell's central longitudinal axis 190.

[0168] It will be appreciated that each of the cell embodiments areintended to have a conventional film label wrapped around the cellcasing. Suitable plastic films for such a label are known in the art andtypically comprise polyvinylchloride. The label can be imprinted withdesired design or indicia. Such labels may be adhesive coated, typicallyat the edges and applied by wrapping around the casing surface.Alternatively, the label can be applied in the form of a sleeve and heatshrunk onto the casing surface. A typical label 12 is shown in FIG. 17.

Separator Fabrication

[0169] In the cell embodiment shown in FIGS. 10-13 the separator 140 canbe conveniently formed from separator material conventionally employedin alkaline cells. For example, typically of cellulose or cellulosic andpolyvinylalcohol fibers. Cell 10 (FIGS. 10-13) may typically be ofcuboid configuration having an overall thickness of about 6 mm, a widthof about 17 mm and a length of between about 35 and 67 mm. A desirableseparator may be composed of a dual layer, the outer layer (facingcathode 110) comprising cellulosic and polyvinylalcohol fibers and theinner layer facing anode 150 comprising a cellulosic material, such asrayon. After the cathode disks 110 are inserted into the casing throughopen end 102 (FIG. 11) separator 140 is inserted so that the outer layerof separator 140 faces the exposed cathode surface.

[0170] Separator 140 for use in the above described flat cell (FIGS.10-13) is conveniently made by rolling a flat separator sheet as shownin FIGS. 14-14B. The near edge 140 a is first folded inwardly in thedirection F1 along a bend line 140 b which is displaced about 3 to 5 mmfrom edge 140 a. A mandrel (not shown) can be placed against theseparator surface 140 d. Then separator far edge 140 c can be wrappedaround the mandrel so that edge 140 c passes around edge 140 a resultingin the partially wrapped configuration shown in FIG. 14A. Far edge 140 cis continually wrapped over the separator surface in the direction F2(FIG. 14A). Edge 140 c can be adhesively secured to the separatorsurface. The bottom separator edge 142 may then be folded inwardly about3 mm over the back surface of the separator as shown in FIG. 14B andadhesively secured thereto. The use of adhesive to secure edge 140 cand/or bottom separator edge 142 can be eliminated by simply foldingbottom separator edge 140 c in the direction F3 and then pressing it toform the bag configuration shown in FIG. 14B. When the folded separatoris inserted into the casing, the cathode shape also helps maintain theseparator in the bag configuration shown in FIG. 14B. The mandrel (notshown) is removed. The resulting wrapped separator 140 (FIG. 14B) is inthe shape of a bag having a closed boundary surface 148 with an open end144 and opposing closed end 143. The separator boundary surface 148defines a cavity 155 for insertion of anode material 150 therein.

[0171] After the cathode disks 110 a are inserted into the cell casing100, the wrapped separator 140 in the configuration shown in FIG. 14B isinserted so that the separator surface faces the exposed cathodesurface. The separator boundary surface 148 forms the anode cavity 155.The anode cavity 155 preferably has an oblong configuration as shown inFIG. 14B. The short dimension of the oblong configuration of cavity 155may typically be between about 2 and 3 mm for a cell 10 of cuboidconfiguration and overall thickness of about 6 mm. Preferably, the longdimension of the oblong configuration is somewhat less than the width ofthe available space within the cell for the total anode cavity 155. Thisresults in a gap 145 between one short side of the separator and cathodedisks 110 a as shown in FIG. 16. Gap 145 is between about 2 and 4 mm,preferably between about 2 and 3 mm. Anode material 150 can then befilled into the anode cavity 155 through the separator open end 143resulting in a casing 100 filled with anode material 150, cathodematerial 110, and separator 140 therebetween, wherein there is a gap 145(void space) between one short side of the separator and the cathode asshown in FIG. 16.

[0172] It has been determined that if desired, additional alkalineelectrolyte solution can now be added to the cell interior by dispensingit directly into gap 145. The additional electrolyte may be dispensedinto gap 145 by inserting a dispensing nozzle directly into said gap. Ina preferred embodiment a small quantity of additional electrolytesolution may be added into gap 145 after the anode 150 and cathode 110are in place in the cell. In an alterative embodiment a portion of theadditional electrolyte may be added to gap 145 after the cathode is inplace but before anode material is inserted into the anode cavity. Thenanode material can be inserted into the cell's anode cavity andthereafter a final quantity of electrolyte may be added into gap 145. Ineither case the additional electrolyte helps to improve anodeutilization (percent anode actives discharged) and overall cellperformance. It can also be a factor tending to retard cell swelling.

[0173] The addition of electrolyte solution to the gap 145 betweenseparator 140 and cathode 110 as above described avoids overflowproblems which can occur if electrolyte is added directly to the anode.For example, if there is initially no gap between separator 140 andcathode 110 and additional electrolyte is added to the cell interiorthere may be an overflow of electrolyte into void space 146 (FIG. 11)above anode 150. Such overflow is undesirable, since it could causewetting along the underside or edge of metal cover 230 after the metalcover 230 is placed in position covering the cell's open end 102. Suchwetting in turn would adversely affect proper laser welding of the metalcover 230 to casing 100.

[0174] If additional electrolyte is added to gap 145, it is recommendedthat it be done in incremental steps. For example, if the additionalelectrolyte is added to gap 145 after the anode 150, cathode 110 andseparator 140 is inserted into the casing, it is recommended that suchadditional electrolyte be added incrementally in a plurality ofdispensing steps allowing for a time lapse, typically of between about 1and 4 minutes between dispenses. This will allow time for eachincremental amount of electrolyte to be absorbed into the anode,separator and cathode reducing the chance of any overflow occurring,which could interfere with proper laser welding of metal cover 230. Ifthe cell is a rectangular 7/5-F6 size cell, typically the amount ofadditional electrolyte solution to be added to gap 145 will be underabout 1 gram. Preferably, in such case the electrolyte can be added inabout four incremental equal dispenses with a time interval of betweenabout 1 to 4 minutes between dispenses to allow sufficient time for theincremental amounts of electrolyte to be properly absorbed into theanode, cathode and separator. Surprisingly, it has been discovered thatwhen the total of said additional electrolyte is dispensed into gap 145,the anode 150 swells sufficiently to expand the anode cavity 155 boundedby separator surface 148. As anode 150 swells the separator surface 148is pushed flush against both anode and cathode, thereby completelyclosing gap 145.

[0175] A performance test of cell 10 made in accordance with theembodiment shown and described with reference to FIGS. 10-16 was madeand is reported in the following examples. The following specificexamples show comparative performance for same size flat alkaline cellsdischarged fresh. The comparative cell (Comparative Example) used thesame size flat cell as shown in FIGS. 10-13 including a conventionalzinc anode and a conventional cathode including MnO₂ and naturalgraphite. The test cells (Examples) also used the same size flat cell asshown in FIGS. 10-13, but employed an anode comprising zinc fines and acathode comprising nickel oxyhydroxide as the cathode active materialand an oxidation-resistant graphite. The fresh cell in each case had athickness of 5.6 mm, a width of 17 mm, and length of 67 mm. (Alldimensions are outside dimensions without a label around the casing,unless otherwise specified.) Casing 100 wall thickness was identical at0.38 mm for all cells tested. The casing 100 for each cell wasnickel-plated cold-rolled steel coated on its inner surface withgraphitic carbon.

Test Cells COMPARATIVE EXAMPLE Zinc/MnO₂ Cell with No Zn Fines

[0176] Test cells having the same rectangular (i.e., cuboid)configuration as shown in FIGS. 10-13 were prepared. The externaldimensions of the cell casings were about 67 mm in length, about 17 mmin width, about 5.6 mm in thickness (i.e., before fresh discharge). Theanode and cathode of the test cells of the Comparative Example had thefollowing compositions: Anode Composition: Wt. % Zinc¹ 60.0 Surfactant²0.10 (RM 510) Electrolyte³ 39.90 (8.5 N KOH) 100.00

[0177] Cathode Composition: Wt. % MnO₂ (EMD)¹ 83.98 Graphite² 10.0(NdG-15) Electrolyte 6.02 (9 N KOH) 100.0

[0178] The cathode and anode of the comparative test cells were balancedsuch that the ratio of the theoretical capacity (in mA-hr) of the MnO₂cathode (based on 370 mA-hr per gram MnO₂) divided by the theoreticalcapacity (in mA-hr) of zinc anode (based on 820 mA-hr per gram zinc) was1.5. The cathode contained about 6.63 grams of MnO₂. The anode containedabout 2.0 grams zinc.

[0179] Fresh test cells were discharged continuously for 1 hour at a 90milliwatt drain rate with 100 milliwatt pulses 5 seconds in durationevery 10 minutes followed by a 3 hour rest period and then repeating thecycle until a cutoff voltage of 0.9 Volts was reached. (This test can beused to simulate using the cell in a typical solid state audio (SSA)player.) The average service life of the comparative test cells was 20.3hours. This test was also performed on test cells stored for 2 weeks atup to 55° C. Average service life of stored test cells was 19.3 hours.On average, the cell casing swelled from an initial thickness of 5.6 mmfor fresh cells before discharge to 5.95 mm after discharge. Thicknessmeasured between outside surfaces of side walls 106 a and 106 b as shownin FIG. 1A.

[0180] Fresh test cells were discharged for 1 hour at a constant drainrate of 125 milliwatts with 300 milliwatt pulses 5 seconds in durationevery 10 minutes, followed by a 3 hour rest period, and then repeatingthe entire cycle until a cutoff voltage of 0.9 Volt was reached. (Thistest can be used to simulate using the cell in a CD or MP3 digital audioplayer.). Average service life of the test cells was 11.2 hours. Thistest was performed on test cells stored for 2 weeks at up to 55° C.Average service life of stored test cells was 10.6 hours.

[0181] Fresh test cells also were discharged continuously for 1 hour ata drain rate of 435 milliwatts with one 1000 milliwatt pulse 20milliseconds in duration every second, followed by a constant drain rateof 5 milliwatts for 1 hour, then a 1 hour rest period, and repeating thecycle until a cutoff voltage of 1.0 Volts was reached. (This test can beused to simulate using the cell in a high power application, forexample, in a personal digital assistant (PDA) having a color display.)Average service life of the test cells was 4.4 hours.

Test Cells EXAMPLE 1 Zinc/NiOOH Cell with 50 wt. % Zn Fines

[0182] Test cells having the same rectangular (cuboid) configuration asshown in FIGS. 10-13 and same size as the test cells of the ComparativeExample were prepared. The anode and cathode of the test cells ofExample 1 had the following compositions: Anode Composition: Wt. % Zinc¹60.0 Surfactant² 0.1 (RM 510) Electrolyte³ 39.9 (8.5 N KOH) 100.0 # Thatis, the zinc fines comprised about 50 wt. % of the total zinc-basedparticles in the anode. The zinc-based particles including the zincfines were alloyed and plated with indium to give a total indium contentof about 200 ppm relative to the total weight of zinc-based particles.

[0183] Cathode Composition: Wt. % NiOOH¹ 85.0 Binder² 1.0 (polyethylene)Graphite³ 8.0 (TIMREX SFG15) Electrolyte 6.0 (7 N KOH) 100.0 #surfacecoating of cobalt oxyhydroxide in the amount of about 4 wt. % of thepure NiOOH. Thus, the actual amount of active NiOOH comprised about85/1.04 = 81.7 percent by weight of the cathode. The cobaltoxyhydroxide-coated nickel oxyhydroxide powder is available from KansaiCatalyst co., Ltd. (Osaka, Japan).

[0184] The cathode and anode of the test cells of Example 1 werebalanced such that the ratio of the theoretical capacity (in mA-hr) ofthe NiOOH (based on 292 mA-hr per gram NiOOH) divided by the theoreticalcapacity (in mA-hr) of zinc (based on 820 mA-hr per gram zinc) was about1.0. The anode included about 2.0 grams of zinc-based particles. Thecathode included about 6.12 grams of cobalt oxyhydroxide-coated nickeloxyhydroxide wherein the cobalt oxyhydroxide content was about 4 wt. %of the total amount of pure NiOOH. The amount of NiOOH in the cathode(on a pure basis) was 6.12/1.04=5.88 grams.

[0185] Fresh test cells were discharged continuously for 1 hour at a 90milliwatt drain rate with 100 milliWatt pulses 5 second long every 10minutes, followed by a 3 hour rest period, and then repeating the cycleuntil a cutoff voltage of 0.9 Volts was reached. Average service lifefor test cells of Example 1 was 23.9 hours. This test also was performedon test cells stored for 2 weeks at up to 55° C. Average service life ofstored test cells was 21.5 hours. On average, the cell casing swelledfrom an initial thickness of 5.6 mm for fresh cells before discharge to5.75 mm after discharge. Thickness measured between outside surfaces ofside walls 106 a and 106 b as shown in FIG. 1A.

[0186] Fresh test cells were discharged at a constant drain rate of 125milliwatts with 300 milliWatt pulses of 5 seconds duration every 10minutes for 1 hour, followed by a 3 hour rest period, and then repeatinguntil a cutoff voltage of 0.9 Volt was reached. Average service life ofthe test cells of Example 1 was 16.0 hours. This test was also performedon test cells stored for 2 weeks at up to 55° C. Average service life ofstored test cells was 14.2 hours.

[0187] Fresh test cells also were discharged continuously for 1 hour ata drain rate of 435 milliwatts with one 1000 milliwatt pulse 20milliseconds in duration every second, then at a constant drain rate of5 milliwatts for 1 hour, followed by a 1 hour rest period, and repeatingthe entire cycle until a cutoff voltage of 1.0 Volts was reached.Average service life of test cells of Example 1 was 5.9 hours.

Test Cells EXAMPLE 2 Zinc/NiOOH Cell with No Zinc Fines

[0188] Test cells having the same rectangular (i.e., cuboid)configuration as shown in FIGS. 10-13 and the same size as the testcells of the Comparative Example were prepared. The cathode and anodehad the same compositions as the test cells of Example 1. The test cellsof Example 2 were identical in every respect to the test cells ofExample 1, except that no zinc fines were included in the anode. Thatis, the zinc-based particles were larger, that is, between −60 mesh and+200 mesh in size.

[0189] Fresh test cells were discharged continuously for 1 hour at a 90milliWatt drain rate with 100 milliWatt pulses of 5 seconds durationevery 10 minutes, followed by a 3 hour rest period, and then, repeatingthe entire cycle until a cutoff voltage of 0.9 Volts was reached.Average service life for the test cells of Example 2 was 23.1 hours.This test also was performed on test cells stored for 2 weeks at up to55° C. Average service life of stored test cells was 22.0 hours. Onaverage, the cell casing swelled from an initial thickness of 5.6 mm forfresh cells before discharge to 5.78 mm after discharge. Thicknessmeasured between outside surfaces of side walls 106 a and 106 b as shownin FIG. 1A.

[0190] Fresh test cells also were discharged at a constant drain rate of125 milliWatts with 300 milliWatt pulses of 5 seconds duration every 10minutes for 1 hour, followed by a 3 hour rest period and then repeatingthe cycle until a cutoff voltage of 0.9 Volt was reached. Averageservice life of test cells of Example 2 was 15.2 hours. This test wasalso performed on test cells stored for 2 weeks at up to 55° C. Averageservice life of stored test cells was 12.7 hours.

Discussion of the Performance Test Results

[0191] The performance test results for the flat zinc/NiOOH cells of theinvention clearly demonstrate an improvement in service life at both lowand high drain rates for test cells with cathodes comprising nickeloxyhydroxide compared to the same size test cells with cathodescomprising manganese dioxide. The improvement is more pronounced athigher drain rates. At relatively low intermittent drain rates, forexample, between 90 and 100 milliWatts, average service life of freshzinc/NiOOH test cells of Example 1 was 23.9 hours versus 20.3 hours forthe same size fresh zinc/MnO₂ test cells of the Comparative Example.This represents an improvement of about 18%. At somewhat higherintermittent drain rates, for example, between 125 and 300 milliwatts,average service life of fresh zinc/NiOOH test cells of Example 1 was16.0 hours versus 11.2 hours for the same size fresh zinc/MnO₂ testcells of the Comparative Example. This represents an improvement ofabout 43%. At even higher intermittent drain rates, for example, between435 and 1000 milliwatts, average service life of fresh zinc/NiOOH testcells of Example 1 was 5.9 hours versus 4.4 hours for the same sizefresh zinc/MnO₂ test cells of the Comparative Example. This representsan improvement of about 34%.

[0192] The test results also show an additional improvement in servicelife, particularly at higher drain rates, for test cells of Example 1having zinc fines in the anode versus the same size test cells ofExample 2 not having zinc fines in the anode. At relatively lowintermittent drain rates, for example, between 90 and 100 milliwatts,the average service life for test cells of Example 1 with 50 wt. % zincfines was 23.9 hours versus 23.1 hours for test cells of Example 2 withno zinc fines. This represents an improvement of about 3%. At somewhathigher intermittent drain rates, for example, between 125 and 300milliwatts, average service life for test cells of Example 1 with 50 wt.% zinc fines was 16.0 hours versus 15.2 hours for the test cells ofExample 2 with no zinc fines. This represents an improvement of about5%.

[0193] The test results further show that average service lives for testcells of Example 1 and Example 2 stored for 2 weeks at up to 55° C.before testing are both greater than that for test cells of theComparative Example when discharged at low to moderate drain rates. Whenstored test cells were discharged at relatively low intermittent drainrates, for example, between 90 and 100 milliwatts, the average servicelife of test cells of both Example 1 and Example 2 was about 22 hours,whereas that for test cells of the Comparative Example was 19.3 hours.When stored test cells were discharged at moderate intermittent drainrates, for example, between 125 and 300 milliwatts, the average servicelife of test cells of Example 1 was 14.2 hours and that for test cellsof Example 2 was 12.7 hours, whereas that for test cells of theComparative Example was only 10.6 hours.

[0194] The test results also show that the amount of deformation of thecell casing by swelling or bulging after discharge of the flat Zn/NiOOHcells of the invention is much less than that for the same size flatzinc/MnO₂ cells. For example, the test cells of both Example 1 andExample 2 expanded by 3% whereas the test cells of the ComparativeExample expanded by about 6% after intermittent discharge at low drainrates (e.g., 90 and 100 mW).

[0195] Although the preferred embodiments of the invention have beendescribed with respect to a flat alkaline cell having an overall cuboidshape (i.e., rectangular parallelepiped), it will be appreciated thatvariations of such overall shape are possible and are intended to fallwithin the concept of the invention. In the case of a flat battery, forexample, having a cuboid shape, the terminal ends of the casing can beslightly tapered either outwardly or inwardly, and yet maintain therectangular configuration. The overall appearance of such a varied shapeis still essentially that of a cuboid and is intended to fall within themeaning of cuboid or the legal equivalent thereof. Other variations tothe overall shape such as slightly altering the angle that the ends ofthe cell can make with any one of the sides of the casing, so that theparallelepiped slightly deviates from strictly rectangular is alsointended to fall within the meaning of cuboid as used herein and in theclaims.

[0196] The present invention desirably is intended to extend to anoverall battery shape that is flat, that is, a side of the outer casingalong the length of the casing is substantially flat. Thus, it shall beunderstood that the term “flat” is intended also to extend to andinclude surfaces that are substantially flat in that their degree ofcurvature may be slight. In particular, the concept of the presentinvention is intended to extend to flat batteries wherein a side of thecasing surface along the length of the casing can have a flat polygonalsurface. The battery can thus have the overall shape of a polyhedronwith all sides of the outer casing being polygonal. The invention isalso intended to extend to batteries wherein a side of the batterycasing along its length has a flat surface and is a parallelogram,wherein the overall shape of the battery is essentially prismatic.

What is claimed is:
 1. A primary alkaline cell comprising a negative anda positive terminal, and an outer casing comprising at least one sidehaving a surface other than a complete cylindrical surface running alonga portion of the length of said casing, said casing having a closed endand an opposing open end, said cell further comprising an anodecomprising zinc and a cathode comprising nickel oxyhydroxide within saidcasing, a separator between said anode and cathode, and an end capassembly sealing the open end of said casing thereby forming a boundarysurface around the cell interior.
 2. The alkaline cell of claim 1wherein said nickel oxyhydroxide is in the form of a powder comprisingnickel oxyhydroxide particles and at least a portion of the surface ofsaid nickel oxyhydroxide particles is coated with cobalt oxyhydroxide.3. The alkaline cell of claim 1 wherein said cathode further comprisesmanganese dioxide in an admixture with said nickel oxyhydroxide.
 4. Thealkaline cell of claim 3 wherein said manganese dioxide comprises anelectrolytic manganese dioxide (EMD).
 5. The alkaline cell of claim 1wherein said cathode further comprises a conductive carbon comprisingoxidation resistant graphite.
 6. The alkaline cell of claim 1 whereinsaid cathode further comprises a conductive carbon comprising betweenabout 2 and 12 percent by weight of said cathode and wherein saidconductive carbon comprises between about 10 and 100 percent by weightof an oxidation resistant graphite.
 7. The alkaline cell of claim 6wherein the oxidation resistant graphite is in the form of particleshaving a mean average particle size between about 3 and 30 microns. 8.The alkaline cell of claim 6 wherein the oxidation resistant graphite isin the form of particles having a mean average particle size betweenabout 5 and 20 microns.
 9. The alkaline cell of claim 8 wherein saidoxidation resistant graphite has a total ash content of less than 0.1percent by weight.
 10. The alkaline cell of claim 9 wherein saidoxidation resistant graphite is in a particulate form having a B.E.T.specific surface area of less than 10 m²/g.
 11. The alkaline cell ofclaim 6 wherein said oxidation resistant graphite has a total ashcontent of less than 0.1 percent by weight, a B.E.T. specific surfacearea of less than 10 m²/g, and an average particle size ranging betweenabout 3 and 20 microns as determined by laser diffraction.
 12. Thealkaline cell of claim 1 wherein the inside surface of said outer casingfaces said cathode and said inside surface has a conductive coatingthereon comprising an oxidation-resistant graphite.
 13. The alkalinecell of claim 1 wherein said casing has at least one substantially flatside.
 14. A primary alkaline cell comprising a negative and a positiveterminal, and an outer casing having a pair of opposing sides runningalong a portion of the length of said casing, said casing having aclosed end and an opposing open end, said cell further comprising ananode comprising zinc and a cathode comprising nickel oxyhydroxidewithin said casing, a separator between said anode and cathode, and anend cap assembly sealing the open end of said casing thereby forming aboundary surface around the cell interior; wherein said cathodecomprises at least one cathode slab having an opening definedtherethrough devoid of cathode material, with at least a portion of theouter surface of said cathode contacting the inside surface of saidouter casing.
 15. The alkaline cell of claim 14 wherein said cathodecomprises a plurality of rectangular shaped cathode slabs; wherein eachof said slabs has a central opening devoid of cathode material; whereinsaid cathode slabs are stacked within the casing so that said openingsdevoid of cathode material form a core, with the outer surface of saidcathode contacting the inside surface of said casing.
 16. The alkalinecell of claim 14 wherein the inside surface of said casing contactingthe outer surface of said cathode slabs has a conductive coating thereoncomprising an oxidation-resistant graphite.
 17. The alkaline cell ofclaim 14 wherein said opposing sides are at least substantially flat.18. The alkaline cell of claim 14 wherein said outer casing is of cuboidshape.
 19. The alkaline cell of claim 14 wherein said nickeloxyhydroxide is in the form of a powder having a mean average particlesize between about 2 and 50 microns.
 20. The alkaline cell of claim 14wherein said zinc is in the form of a powder having a mean averageparticle size between about 1 and 250 microns.
 21. The alkaline cell ofclaim 14 wherein at least 5 percent by weight of the total zinc in theanode comprises zinc fines of dimensions suitable to pass through astandard 200 mesh screen having square openings of 0.075 mm.
 22. Thealkaline cell of claim 14 wherein at least 10 percent by weight of thetotal zinc in the anode comprises zinc fines of dimensions suitable topass through a standard 200 mesh screen having square openings of 0.075mm and said total zinc in the anode further comprises zinc particles oflarger size than said zinc fines so that the mean average particle sizeof said total zinc is between about 75 and 340 microns.
 23. The alkalinecell of claim 14 wherein at least 10 percent by weight of the total zincin the anode comprises zinc fines of dimensions suitable to pass througha standard 200 mesh screen having square openings of 0.075 mm, whereinthe mean average particle size of said zinc fines is between about 1 and75 microns; and said total zinc in the anode further comprises zincparticles of larger size than said zinc fines so that the averageparticle size of said total zinc is between about 75 and 340 microns.24. The alkaline cell of claim 14 wherein at least 50 percent by weightof the total zinc in the anode comprises zinc fines of dimensionssuitable to pass through a standard 200 mesh screen having squareopenings of 0.075 mm and said total zinc in the anode further compriseszinc particles of larger size than said zinc fines so that the meanaverage particle size of said total zinc is between about 75 and 200microns.
 25. The alkaline cell of claim 14 wherein at least 50 percentby weight of the total zinc in the anode comprises zinc fines ofdimensions suitable to pass through a standard 200 mesh screen havingsquare openings of 0.075 mm, wherein said zinc fines has a mean averageparticle size of between about 1 and 75 microns, and said total zinc inthe anode further comprises zinc particles of larger size than said zincfines so that the mean average particle size of said total zinc isbetween about 75 and 200 microns.
 26. The alkaline cell of claim 14wherein at least 5 percent by weight of the total zinc in the anodecomprises zinc fines of dimensions suitable to pass through a standard325 mesh screen having square openings of 0.045 mm.
 27. The alkalinecell of claim 14 wherein at least 10 percent by weight of the total zincin the anode comprises zinc fines of dimensions suitable to pass througha standard 325 mesh screen having square openings of 0.045 mm, and saidtotal zinc in the anode further comprises zinc particles of larger sizethan said zinc fines so that the mean average particle size of saidtotal zinc is between about 35 and 314 microns.
 28. The alkaline cell ofclaim 14 wherein at least 10 percent by weight of the total zinc in theanode comprises zinc fines of dimensions suitable to pass through astandard 325 mesh screen having square openings of 0.045 mm, whereinsaid zinc fines has a mean average particle size between about 1 and 35micron, and said total zinc in the anode further comprises zincparticles of larger size than said zinc fines so that the mean averageparticle size of said total zinc is between about 35 and 314 microns.29. The alkaline cell of claim 14 wherein at least 50 percent by weightof the total zinc in the anode comprises zinc fines of dimensionssuitable to pass through a standard 325 mesh screen having squareopenings of 0.045 mm, and said total zinc in the anode further compriseszinc particles of larger size than said zinc fines so that the meanaverage particle size of said total zinc is between about 35 and 125microns.
 30. The alkaline cell of claim 14 wherein at least 50 percentby weight of the total zinc in the anode comprises zinc fines ofdimensions suitable to pass through a standard 325 mesh screen havingsquare openings of 0.045 mm, wherein said zinc fines has a mean averageparticle size between about 1 and 35 microns, and said total zinc in theanode further comprises zinc particles of larger size than said zincfines so that the mean average particle size of said total zinc isbetween about 35 and 125 microns.
 31. The alkaline cell of claim 14wherein said nickel oxyhydroxide further comprises a bulk chemicaldopant selected from the group consisting of aluminum, magnesium,cobalt, zinc, gallium, indium, and any mixture thereof.
 32. The alkalinecell of claim 14 wherein said nickel oxyhydroxide comprises a nickeloxyhydroxide selected from the group consisting of beta-nickeloxyhydroxide and gamma-nickel oxyhydroxide and mixtures thereof.
 33. Thealkaline cell of claim 14 wherein said nickel oxyhydroxide is in theform of a powder comprising particles and at least a portion of thesurfaces of said nickel oxyhydroxide particles is coated with cobaltoxyhydroxide.
 34. The alkaline cell of claim 14 wherein said cathodecomprises between about 80 and 95 percent by weight nickel oxyhydroxide.35. The alkaline cell of claim 14 wherein said cathode further compriseselectrolytic manganese dioxide in an admixture with said nickeloxyhydroxide.
 36. The alkaline cell of claim 14 wherein said cathodefurther comprises a conductive carbon comprising oxidation resistantgraphite.
 37. The alkaline cell of claim 36 wherein said oxidationresistant graphite has a total ash content of less than 0.1 percent byweight.
 38. The alkaline cell of claim 37 wherein said oxidationresistant graphite is in a particulate form having a B.E.T. specificsurface area of less than 10 m²/g.
 39. The alkaline cell of claim 38wherein said oxidation resistant graphite has an average particle sizeranging between about 3 and 30 microns as determined by laserdiffraction.
 40. The alkaline cell of claim 39 wherein said oxidationresistant graphite has an average particle size ranging between about 5and 20 microns as determined by laser diffraction.
 41. The alkaline cellof claim 36 wherein said oxidation resistant graphite has a total ashcontent of less than 0.1 percent by weight, a B.E.T. specific surfacearea of less than 10 m²/g, and a mean average particle size rangingbetween about 5 and 20 microns as determined by laser diffraction. 42.The alkaline cell of claim 39 wherein the oxidation resistant graphitehas a high degree of crystallinity, characterized by having a value forcrystallite size, along the crystallographic “c”-axis direction, Lc, ofgreater than 1500 Angstrom and a d₀₀₂ lattice constant of less than3.356 Angstrom.
 43. The alkaline cell of claim 39 wherein the oxidationresistant graphite has a value for crystal lattice defect ratio of lessthan about 0.15, wherein said defect ratio is defined as the ratio ofthe integrated area underlying the “D” band centered between 1330 and1360 cm⁻¹ to the integrated area underlying the “G” band centeredbetween 1570 and 1580 cm⁻¹ in the first order laser Raman absorptionspectrum.
 44. The alkaline cell of claim 36 wherein said oxidationresistant graphite is available under the trade designation TIMREX SFGgraphite powder.
 45. The alkaline cell of claim 14 wherein said cathodefurther comprises a conductive carbon comprising between about 2 and 12percent by weight of said cathode and wherein said conductive carboncomprises between about 10 and 100 percent by weight of an oxidationresistant graphite.
 46. The alkaline cell of claim 36 wherein theoxidation resistant graphite is in the form of particles having a meanaverage particle size between about 3 and 30 microns.
 47. The alkalinecell of claim 36 wherein the oxidation resistant graphite is in the formof particles having a mean average particle size between about 5 and 20microns.
 48. The alkaline cell of claim 14 wherein the inside surface ofsaid outer casing faces said cathode and said inside surface has acoating thereon comprising an oxidation-resistant graphite.
 49. Thealkaline cell of claim 14 wherein said anode and cathode furthercomprises an electrolyte comprising an aqueous solution of potassiumhydroxide.
 50. A primary alkaline cell comprising a negative and apositive terminal, and an outer casing having a pair of opposing sidesrunning along a portion of the length of said casing; said casing havinga closed end and an opposing open end; said cell further comprising ananode comprising zinc and a cathode comprising nickel oxyhydroxidewithin said casing, a separator between said anode and cathode, and anend cap assembly sealing the open end of said casing; wherein thecathode comprises at least one cathode slab having an opening definedtherethrough devoid of cathode material, with at least a portion of theouter surface of said cathode contacting the inside surface of saidcasing; wherein said cell comprises a vent mechanism located on saidboundary surface, wherein said vent mechanism activates to release gaspressure from within the cell as said gas pressure rises, said ventmechanism comprising a first rupture zone comprising a groove on saidboundary surface, said groove defining an underlying material regionthinner than the average thickness of said boundary; and a secondrupture zone on said boundary surface, wherein the first zone ruptureswhen gas pressure within the cell rises to a first pressure level andsaid second zone ruptures when gas pressure within the cell rises to asecond pressure level being higher than said first pressure levelallowing gas from within the cell to escape from the cell interiorthrough said ruptures.
 51. The alkaline cell of claim 50 wherein thecathode comprises a plurality of rectangular shaped cathode slabs;wherein each of said slabs has a central opening devoid of cathodematerial; wherein said cathode slabs are stacked within the casing sothat said openings devoid of cathode material form a core, with theouter surface of said cathode contacting the inside surface of saidcasing.
 52. The alkaline cell of claim 50 wherein said first and secondrupture zones are spaced apart on said boundary surface.
 53. Thealkaline cell of claim 50 wherein said first rupture zone ruptures whengas pressure within the vessel rises to a pressure between about 250 and800 psig (1724×10³ and 5515 ×10³ pascal gage).
 54. The alkaline cell ofclaim 50 wherein said first rupture zone ruptures when gas pressurewithin the vessel rises to a pressure between about 400 and 800 psig(2758×10³ and 5515×10³ pascal gage).
 55. The alkaline cell of claim 50wherein said second rupture zone ruptures when gas pressure within thevessel reaches a pressure between about 800 and 2500 psig (5515×10³ and17235×10³ pascal gage).
 56. The alkaline cell of claim 50 wherein thesecond rupture zone comprises a laser weld within a portion of saidboundary surface.
 57. The alkaline cell of claim 50 wherein said grooveis formed by stamping said boundary surface.
 58. The alkaline cell ofclaim 56 wherein said end cap assembly comprises a metal cover and saidlaser weld is formed between said casing and a said metal cover fittedwithin the open end of said casing thereby closing said open end. 59.The alkaline cell of claim 50 wherein said end cap assembly comprises ametal cover and said laser weld is formed between the inside surface ofsaid casing and the edge of a metal cover fitted within the open end ofsaid casing thereby closing said open end.
 60. The alkaline cell ofclaim 56 wherein said laser weld is formed by a Nd:Yag laser and saidlaser weld ruptures when gas pressure within the cell rises to a levelof between about 800 and 2500 psig (5515×10³ and 17235×10³ pascal gage).61. The alkaline cell of claim 58 wherein said metal cover is arectangular metal plate.
 62. The alkaline cell of claim 58 wherein saidmetal cover is a rectangular plate having an aperture therethrough. 63.The alkaline cell of claim 59 wherein said end cap assembly comprisessaid metal cover, a terminal end plate, an insulating seal member, andan elongated electrically conductive member having a portion thereofpassing through both said insulating seal member and said metal cover,wherein said conductive member is electrically connected to saidterminal end plate.
 64. The alkaline cell of claim 63 wherein saidelectrically conductive member is electrically connected to said anode.65. The alkaline cell of claims 64 wherein a portion of said elongatedconductive member penetrates into said anode and functions as an anodecurrent collector.
 66. The alkaline cell of claim 63 wherein said endcap assembly further comprises an electrically insulating member betweensaid terminal end plate and said metal cover thereby insulating saidterminal end plate from said metal cover.
 67. The alkaline cell of claim66 wherein said electrically insulating member between said terminal endplate and said metal cover comprises plastic material.
 68. The alkalinecell of claim 66 wherein said electrically insulating member betweensaid terminal end plate and said metal cover comprises paper material.69. The alkaline cell of claim 66 wherein said terminal end plate has acentral area of smaller thickness than the average thickness of said endplate, wherein said elongated conductive member is welded by electricalresistance welding to said terminal end plate at said central area. 70.The alkaline cell of claim 63 wherein sealant material comprisingasphalt is applied between at least a portion of the surface of saidelongated conductive member and said metal cover to prevent leakage ofalkaline electrolyte therethrough.
 71. The alkaline cell of claim 50wherein at least a portion of said central opening within said cathodeslabs forms a cavity for housing said anode.
 72. The alkaline cell ofclaim 71 wherein said cavity has an oblong configuration.
 73. Thealkaline cell of claim 50 wherein the cell comprises an alkalineelectrolyte comprising an aqueous solution of an alkali metal hydroxideselected from the group consisting of potassium hydroxide, sodiumhydroxide, lithium hydroxide, and mixtures thereof.
 74. The alkalinecell of claim 73 wherein said alkaline electrolyte further compriseszinc oxide.
 75. The alkaline cell of claim 50 wherein said opposingsides are at least substantially flat.
 76. The alkaline cell of claim 50wherein said cell has an overall thickness of between about 5 and 10 mm,wherein said overall thickness is defined as the distance between theoutside surface of opposing sides of said casing defining the shortdimension of said casing.
 77. The alkaline cell of claim 50 wherein thecasing comprises metal having a wall thickness of between about 0.30 mmand 0.50 mm.
 78. The alkaline cell of claim 50 wherein the casingcomprises metal having a wall thickness of between about 0.30 mm and0.40 mm.
 79. The alkaline cell of claim 50 wherein said casing comprisessteel.